lmemsm

I covered SDL based applications. Now, I'd like to cover FLTK based applications for desktops and/or productivity.

While there isn't as much FLTK application development going on as I would like, there are some projects that specialize in using FLTK. TinyCore Linux is probably the most well-known Linux project that uses several FLTK applications. Nanolinux is based on TinyCore but uses nano-x as a lightweight alternative to X Windows. The developer of NanoLinux uses mainly FLTK applications and has modified and updated several FLTK applications to give them new life. He's also created some of his own where good alternatives did not exist. The Equinox Desktop Environment also uses FLTK, but it typically requires another EDE specific library along with FLTK support. Also, EDE users don't always look for FLTK applications for their desktops. Some will typically use anything they consider lightweight (whether it really is lightweight or not). A few mobile devices use FLTK as their main GUI. Users of those systems have developed some interesting applications for their devices.

There are several versions of FLTK. Applications may work with one version and not another. I've spent a lot of time searching for applications and porting applications to the latest version. I did try to update the FLTK software links list at the official FLTK web site with information on what worked with the latest version of FLTK and with information on newer FLTK applications, but was unable to add some of the newer, more interesting FLTK applications out there. So, this is my definitive list at this point in time of the best FLTK applications available. For more information on FLTK and applications, see also http://www.distasis.com/cpp/scrlib.htm#fltk

I'm sure I haven't covered everything and as mentioned, check NanoLinux and Tiny Core Linux for more FLTK based applications. I'm always looking for new, portable, lightweight FLTK based applications. If you know of something I may have missed or you're working on a new FLTK based project, please contact me.

Utilities

xdiskusage
Shows disk usage. Works on POSIX systems with du command. I have patches to port this to Windows.
http://xdiskusage.sourceforge.net/

flcalc
Calculator. The FLTK web site link includes information on some of my patches to get it to build successfully as well as link to the original source code.
https://www.fltk.org/links.php?V47+Qflcalc

fldiff
Graphical diff program.
https://www.fltk.org/links.php?V227+Qfldiff

xRecurseDiff
Traverse directories and find file differences. Some of it was based on fldiff.
https://sourceforge.net/projects/xrecursediff/

fpwdman
Password manager. I ported this to work with the latest versions of FLTK and tinyxml2. It's interesting, but at this point, I'd prefer a Keepass compatible password manager. I'm looking into chkpass as a lightweight alternative for password management.
https://sourceforge.net/projects/fpwdman/

Communications

prozgui for prozilla
Fast file downloader. I have patches for building and to port this to Windows. I use a version based on GNU GPLv2 development instead of the GNU GPLv3 development.
https://launchpad.net/prozgui

flchat
IRC client. Based on MegaIRC, but with a lot of cleanup. I have patches to add gettext/libintl support. This is the best option for IRC using FLTK that I've found to date.
https://sourceforge.net/projects/nanolinux/

kashyyyk
This has good potential as a threaded IRC client. It can handle multiple connections. I could get it to build on Windows either with patches or a build of MinGW with POSIX instead of Windows native threading support. It needs some work on storing IRC connections. Doesn't seem to remember any connections once you leave the application.
https://sourceforge.net/projects/kashyyyk/?source=directory

Browsers

There are two webkit based browsers for FLTK. That's great news for FLTK applications users. What's not so great is that they don't port well to non-POSIX systems. If you want the most lightweight webkit based browser (and webkit browsers are not by nature lightweight), I'd go with either of these options instead of the many other webkit ports out there.

Netrider
I was able to get the original version of netrider to port to Windows and a Windows version of it is available at Sourceforge. However, when I upgraded the version of my MinGW compiler, I was no longer able to build netrider. Seems the webkit developers took some shortcuts in the older code that really weren't up to C++ standards. Netrider upgraded to a later version of webkit (which fixed the compiler issue), but the newer version was never ported to Windows.
https://sourceforge.net/projects/netrider/

Fifth
This was never ported to Windows although it might be easier to port that the latest version of Netrider. It uses makefiles created by the developer instead of cmake.
http://fifth-browser.sourceforge.net/

DPlus
When people talk about FLTK based web browsers, Dillo always comes up. However, Dillo is the opposite of what I think of when I think about portable code. One developer decided to fork Dillo and make it more structured and easier to port. He's really done a wonderful job on cleaning up the code. I'd recommend this browser over Dillo if you're interested in doing anything with the source code or need a lightweight HTML viewer for FLTK. DPlus is also the lightest browser I could find that could display output from diffh properly. Most console browsers like lynx had trouble rendering the color differences in the output.
https://sourceforge.net/projects/dplus-browser/

DPlusUI
While this is meant as a utility rather than a web browser, I used DPlus as the starting point for my HTML/CSS based dialog replacement.
http://www.distasis.com/cpp/lmbld.htm#lmbldui

Mail clients

flmail
The developer of Nanolinux wrote a nice, basic, stable e-mail client. (He also reused part of my Open Source POP3 e-mail code.)
https://sourceforge.net/projects/nanolinux/

Postoffice
This has a lot of potential. It took a long while to get it to build with the latest version of FLTK and it's still kind of buggy. I also needed to update helper libraries fl_toggletree and fleditor to work with the latest FLTK. The interface is a lot of like sylpheed and foxmail. I would love to see some new development on this and would be happy to help update it.
ftp://linuxmafia.com/faq/Mail/muas.html

hermail
There was a nice, very basic, stable e-mail client at Sourceforge. Doesn't appear to be available from there any longer.

RSS reader
Gautier's RSS reader
This one has a lot of potential. It has an attractive user interface that's easy to work with. However, it does have the ability to sort RSS posts at this point at time. It needs to be used in conjunction with a script and tools like curl to download the RSS feeds. It's basically just a reader. It uses SQLLite to store the RSS data so it can potentially provide fast access to RSS posts. I'd love to see some further development done on this project.
https://michaelgautiertechnology.wordpress.com/2015/10/09/gautier-rss-app-technology/

AV

flvlc
Cross-platform VLC based media player.
https://sourceforge.net/projects/flvlc/

flxine
Unfortunately, this only works on POSIX systems so far. I have been able to build it on Cygwin as well as BSD and several Linux systems. This might be portable to more platforms using nano-x (and possibly SDL as the backend for nano-x), but I did not get very far in investigating this option. It is my favorite xine front end and is more lightweight than many of the other xine front end options. It provides a variety of features including a nice visualization component for use while playing music.
https://www.fltk.org/links.php?V199+Qflxine

Audio

flrec
Simple Audio recorder and player based on Sox. I have done some work to port this one to Windows.
http://matteolucarelli.altervista.org/flrec/index_en.htm

Fl_MIDIKeyboard
Midi keyboard.
https://github.com/ncassetta/Fl_MIDIKeyboard

PaulStretch
Audio effects program to stretch sounds.
http://hypermammut.sourceforge.net/paulstretch/

APCStudio
After more than one try to get this to build with the latest version of FLTK, I finally managed to get this working. It's a nice, lightweight audio editor. It doesn't display multiple tracks like Audacity. It does not have good support for playing or recording wave files. It's basically just a wave file editor. Was considering using libsox or another Open Source sound library to add support for playing wave files.
https://www.fltk.org/links.php?V204+Qapcstudio

There is a fork of FLTK called NTK. It isn't as portable as FLTK and requires POSIX/X Windows support. A suite of audio applications were created with it.

Graphics

Rendera
This is a great, lightweight graphics editor. I really like this one.
https://github.com/Mortis69/rendera
https://sourceforge.net/projects/rendera/

PhotoColoring
Specialized graphics editor for coloring old photos.
https://sourceforge.net/projects/photocoloring/

Other graphics and drawing options include Antipaint and Cinepaint. Antipaint was updated to work with the lastest version of FLTK and to improve portability by the developer of Nanolinux. You'll find it at the Nanolinux web site. At one point Cinepaint decided to port their project from GTK to FLTK. You'll find some older versions with some FLTK support and utilities. However, the FLTK port is not actively developed.

Productivity

fltdj
The Daily Journal is a Personal Information Manager (PIM). It has several nice features including the ability to set alarms to remind you of appointments. I use one of the older versions (0.7) which ports well to newer versions of FLTK and, per my recommendation, so does NanoLinux.
https://www.fltk.org/links.php?V386+Qfltdj

tux_todo
Simple todo list. Haven't used it in a while, but if you're looking for a todo list program, it's an option.
https://www.fltk.org/links.php?V76+Qtux_todo

PDF/Ebook Readers

SpRead
A functional, basic PDF viewer and archived image viewer. I really like this one. It requires a compiler with later C++ support to build. Needs minimal dependencies, mainly libarchive and poppler. While poppler isn't as fast as mupdf at rendering, this still works pretty fast.
https://github.com/muranoya/SpRead

flviewer
Image viewer with plugin support for mupdf and poppler.
https://github.com/madrat-/flviewer

There's also flaxpdf which is optimized for efficiency and uses mupdf. However, it's not at all portable to non-POSIX systems.

I tried BDReader as well but there are a lot of dependencies involved in building this.
https://sourceforge.net/projects/bdreader/

File managers

I don't use any of these and personally prefer the SDL based file manager mentioned earlier. However, they're nice lightweight GUI file managers. I'm sure there are a few others not mentioned below as well.

mfm
http://sg67.altervista.org/mfm/

fluff
https://sites.google.com/site/lockmoorecoding/downloads/fluff

Other POSIX only FLTK applications

flsynclient
GUI front end for synaptics touchpad controls.
http://matteolucarelli.altervista.org/flsynclient/index_en.htm

alsamixergui
FLTK ALSA Mixer front end.
https://packages.debian.org/jessie/alsamixergui

Editors

I've yet to find a FLTK based editor I really like. For now, I'm still using SciTE, Fxite and nano.

I would love to find a Scintilla based FLTK editor. The closest I've found is https://github.com/cyantreeguo/Fl_Scintilla

There are many FLTK editor controls out there, including the one used by Postoffice.

Nanolinux offers flwriter.

The most interesting editor option I've found so far is fldev. There's a link to the original at https://www.fltk.org/links.php?V235+Qfldev and further development by the developer of NanoLinux at https://sourceforge.net/projects/fldev/ The main drawback is that it only opens one file at a time. The original author began to add support to use it as a debugger in conjunction with gdb. I'd love to get that support working properly and in a cross-platform manner and have been experimenting with it as time allows.

flabc is an editor specifically designed for ABC notation, but it appears to be a stable, well-written editor and may be useful for other editing purposes.