BackBox Linux is a new penetration testing/security Linux distribution.
It's based on Ubuntu 11.04 and has something like 77 tools included (with
more in the repository).
It's different from BackTrack in that it is a truly open source project that
anybody can contribute to and is not backed by a corporate entity. All
previous versions can be downloaded and are supported. Also, you can
add the BackBox PPA to your favorite Debian-based distro and install
the BackBox packages on it.
I've been messing around with it a bit, and it's worth a look. BackBox 3
should be coming out this September.