I use SourceTree too - I tried to use GitHub for Mac, but it requires a 64-bit Mac, which I don't have. Guess they're just too cool for those old systems...it would be nice if they made that clear upfront, so people could save the wasted download. [EDIT - they do point out 64-bit 10.6+ in the header, it's just gray and I didn't notice it.]

Even if SourceTree is a bit more "map the git commands onto buttons" it works well for me when I'm not on my Linux system, and the fact that it handles Hg as well is a nice bonus.