BusyBox combines tiny versions of many common UNIX utilities into a single small executable. It provides minimalist replacements for most of the utilities you usually find in GNU fileutils, shellutils, etc. The utilities in BusyBox generally have fewer options than their full-featured GNU cousins; however, the options that are included provide the expected functionality and behave very much like their GNU counterparts. BusyBox provides a fairly complete POSIX environment for any small or embedded system.
uClibc (µClibc) is a C library for developing embedded Linux systems. It is much smaller then the GNU C Library, but nearly all applications supported by glibc also work perfectly with uClibc. Porting applications from glibc to uClibc typically involves just recompiling the source code. uClibc even supports shared libraries and threading. It currently runs on standard Linux and MMU-less Linux (also known as µClinux) systems with support for ARM, i386, h8300, m68k, MIPS, mipsel, PowerPC, SH, SPARC, and v850 processors.