I generally have one of Fink, MacPorts, Homebrew installed. Most often for a single, small, and trivial package. I've found that all my day-to-day software exists in OS X versions.
So, which non-OS X unix software do you find is required, interesting, or otherwise always on your computer.
I'm looking to broaden my horizons. I have enough unix/linux experience to not be afraid, I just haven't found a good use-case yet.
For added clarity, I'm not looking for anything already installed with OS X. So, please, no ssh, vi, etc, unless you explain the reason you need a different version.