lmemsm

Most systems (other than BSD based ones) use GNU's core utilities. It's used by most Linux distributions. Cygwin, MinGW and gnuwin32 run ported versions of the GNU applications as well. Even Microsoft's SFU/SUA included some of the GNU utilities. However, the GNU core utilities are typically more bloated and have more feature creep than other versions of standard Unix utility programs. BSD systems have their versions of core utilities. The latest version of Minix has adopted the BSD utilities. They tend to be less bloated than the GNU versions, but are still more bloated than other options out there. The BSD utilities also tend toward adding new features similar to but not to the same extent as the GNU utilities. Also, some of their utilities aren't as well optimized as the GNU versions. Busybox seems like the most viable option for a lightweight but still comprehensive version of core utilities. I'm currently using it on my Debian system instead of the GNU core utilities. Toybox is a similar alternative to Busybox. It has a better license option than Busybox, but it's lacking some features and tools that Busybox has.

Here are some links to core utility collections:

Earlier Minix alternatives
http://www.minix-vmd.org/cgi-bin/raw/source/std/1.7.5/src/commands/simple/
Earlier versions of Minix put together an interesting collection of lightweight utilities from various sources.

Busybox
https://busybox.net/
Windows ports of Busybox:
https://frippery.org/busybox/
https://github.com/rmyorston/busybox-w32
https://github.com/realthunder/busybox-w32

Toybox
http://landley.net/toybox/

Heirloom Project
http://heirloom.sourceforge.net/
Based on traditional implementations of standard Unix utilities. Not very portable to non-POSIX systems. Not as bloated as GNU or BSD core utilities.

OBase
https://github.com/chneukirchen/obase
Port of OpenBSD userland to Linux.

SBase
http://git.suckless.org/sbase
This started out as a discussion on one of the suckless.org mailing lists of how to write efficient core utilites that weren't all part of one executable like Busybox or Toybox. Some good examples were posted and the project was started. Then project development was quiet for a while. The project became active again and one of the main goals besides efficiency was UTF-8/internationalization support. Looks like they've borrowed some UTF-8 support concepts (such as Runes) from Plan 9. It's not designed to be portable to non-POSIX systems. However, it does look like they've covered replacing most of the basic core utilities with lightweight, efficient versions.

Other alternatives:
https://github.com/jbruchon/elks/tree/master/elkscmd
https://github.com/EtchedPixels/FUZIX/tree/master/Applications
https://github.com/Orc/bin
https://github.com/eltanin-os/utilchest
https://github.com/eltanin-os/cbase
https://github.com/rofl0r/hardcore-utils
http://git.musl-libc.org/cgit/noxcuse/tree/
https://github.com/pikhq/pikhq-coreutils
http://www.fefe.de/embutils/
http://skarnet.org/software/s6-portable-utils/
https://github.com/dimkr/lazy-utils
https://github.com/arsv/minitools
http://git.suckless.org/ubase
https://github.com/dcantrell/bsdutils
https://github.com/cheusov/nbase
https://github.com/rswier/swieros
https://github.com/minoca/swiss
https://github.com/mentos-team/MentOS/tree/master/programs