I've been using GitX for months and it's pretty good, but it still buggy at times. I've been using the GitX (L) fork, since the original GitX doesn't seem to be actively maintained anymore. It's available here: http://gitx.laullon.com/
A few days ago a co-worker turned me on to SourceTree (http://www.sourcetreeapp.com/), which is also free and developed by Atlassian. I've got to say it's a bit more polished and so far it looks like it might be replacing GitX for me.
I just found source tree myself and really enjoy it. The best feature for me is how it allows you to view diffs of all your changes as you write your commit message, that makes it easy to include good detail on all changes.
Also, it's great that Source Tree supports multiple projects. I tried Tower a while ago and it only allowed you to have one project open at a time. Maybe that's changed now, but it's also not free.
I don't agree; Tower is pretty good (and pretty pretty), but SourceTree is siginificantly more performant and full-featured. It's much faster than Tower with large repos, and can open more than one repo at the same time, supports Git submodules (not to mention Hg and svn, though I don't need/use those).
I bought SourceTree for $60 and was completely satisfied, but since then Atlassian bought it and made it free. I would say SourceTree is the best Git client for Mac (or any platform).
