I've just noticed that Github have now got a Mac client for managing repository on the github.com domain. Its by no way comparable to Tower as from what I can tell you cannot hook up just any old git repos. Worth a look though if you manage several open source projects on there.
While I like projects like this that makes git easier to start to I still think git needs you start to understand it on command line. Git is so much more than commit/update svn standard. Did you try gitx ???
You can hook up any repo, whether connected to GitHub or not. Just drag the repo's working tree into the Repositories list. It works like a champ. This is a huge deal for my students who are just starting out with Git and don't need a ton of features, just basic history and reverting, and eventually some minor branching.