Standalone rescue shell with tons of builtin utilities

 BusyBox combines tiny versions of many common UNIX utilities into a single
 small executable. It provides minimalist replacements for the most common
 utilities you would usually find on your desktop system (i.e., ls, cp, mv,
 mount, tar, 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-static provides you with a statically linked simple stand alone shell
 that provides all the utilities available in BusyBox. This package is
 intended to be used as a rescue shell, in the event that you screw up your
 system. Invoke "busybox sh" and you have a standalone shell ready to save
 your system from certain destruction. Invoke "busybox", and it will list the
 available builtin commands.