I have been looking for a Mac OS X Git client, and these are the results of my initial search, I will try and pad out the descriptions of each as I become more familiar with them, and may even put together a feature matrix if I have the time. For now, here are my initial thoughts.
Free download - Still in Beta
Powerful, visually appealing, native OS X Git client. A few stability issues when I used it, but it is still in Beta.
Basic functions, clean native interface. Had a few stability issues with this application, I didn't feel I could trust it with my actual repositories.
Fast, clean, native OS X interface. This client would probably suffice for 90% of my daily Git use, although I would still have to use the command line occasionally. No built in diff viewer, or visual revision tree.
One of the oldest OS X Git clients, with some really nice features. The History viewer is easy to use, and the best amongst all the clients I have seen. Features again, probably suffice for 90% of my daily Git use. The interface is not the best overall, but it doesn't take long to adapt to it.
Free for non-commercial use - Version 1.5
SmartGit is not a native OS X application, however it largely looks and feels as though it is. This is the most powerful client I have used, and when using it, i never have to fall back on the command line. The stability of SmartGit is very good, i have been testing it for some time, and had no problems at all, and would trust it for my day to day work.
SourceTree is an impressive native Git and Mercurial client. It is feature rich with the best history viewer I have yet seen. It has no side by side built in diff, but integrates easily with all the popular third parties. I found that the interface can sometimes feel crowded, but this is not a big issue.
Free beta - Version 0.9.40
Tower is fairly new and still in beta, but from what i have seen so far, i am very impressed. Although it's still in beta, it out performs all other clients in terms of stability, features and its interface (excluding perhaps SmartGit). There are a couple of areas where I feel Tower falls short of SmartGit. Firstly, there appears to be no way to view the history of a specific file. Secondly, I much prefer a side by side diff viewer like the one packaged with SmartGit, although Tower seamlessly and effortlessly integrates with third party diff and merge tools such as XCode FileMerge, Araxis, and Kaleidoscope. In my opinion, Tower is visually stunning with one exception, I really don't like the application icon.
Free beta - Version 0.3.3 (22)
Just tried this new Git client, which I had high hopes for, but in its current state it's very disappointing. As it stands, the user interface is poor, it has no built in diff, and no branch history, and numerous other basic features are missing. There also seem to be major bugs, for example I couldn't work out how to view any branch other than the current one, or even be able to switch branches!
My favourite so far? it has to be SmartGit, although its not the prettiest, it is intuitive, provides all the features I need on a daily basis, and inspires confidence with its stability I am actually now using Tower as my git client of choice, it's still only in beta which gives me high hopes for the future.
Please let me know if I have missed any clients off the list.