The Daddy of them all for people who want to fully take advantage of Unix being underneath the hood of OSX, MacPorts
Using MacPort you can install all sorts of Unix goodies on to your Mac to use on the command line it really makes OSX what I like to say “All the power of Unix with some pretty”. I install it then
sudo port install XXX
and I off to the races installing command line tools like subvernsion, git and many others.
All and all even though it takes forever on the initial install and a Unix Admin this is by far the most important addon for my Mac and makes OSX ideal for doing the job.
Edit: A co-worker just showed me HomeBrew. I think I might be trying that on the new machine to see how I like it.