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 fileutils, ShellUtils, findutils, textutils, grep, gzip, tar, etc. BusyBox provides a fairly complete POSIX environment for any small or embedded system. 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 has been written with size-optimization and limited resources in mind. It is also extremely modular so you can easily include or exclude commands (or features) at compile time. This makes it easy to customize your embedded systems. To create a working system, just add /dev, a kernel, and an editor, such as NanoEditor, e3, or elvis-tiny. For a really minimal system, you can even use the busybox shell (not Bourne compatible, but very small and quite usable). 

Web page is at http://www.busybox.net/