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From the Busybox README:
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 bzip2, coreutils, file, findutils, gawk, grep, inetutils, modutils, net-tools, procps, sed, shadow, sysklogd, sysvinit, tar, util-linux, and vim. The utilities in BusyBox often have fewer options than their full-featured cousins; however, the options that are included provide the expected functionality and behave very much like their larger counterparts.
BusyBox has been written with size-optimization and limited resources in mind, both to produce small binaries and to reduce run-time memory usage. Busybox is also extremely modular so you can easily include or exclude commands (or features) at compile time. This makes it easy to customize embedded systems; to create a working system, just add /dev, /etc, and a Linux kernel. Busybox (usually together with uClibc) has also been used as a component of "thin client" desktop systems, live-CD distributions, rescue disks, installers, and so on.
BusyBox provides a fairly complete POSIX environment for any small system, both embedded environments and more full featured systems concerned about space. Busybox is slowly working towards implementing the full Single Unix Specification V3 (http://www.opengroup.org/onlinepubs/009695399/), but isn't there yet (and for size reasons will probably support at most UTF-8 for internationalization). We are also interested in passing the Linux Test Project.