SourceTree is frequently recommended on Stack Overflow, has good reviews on the App Store, and seems to tie with Tower for Mac Git GUI of choice.

Atlassian, maker of cost-hosting service Bitbucket, has acquired SourceTree, a Mac client for Git and Mercurial distributed version control systems and Subversion source control.

And today, Atlassian is buying SourceTree, a popular client for Git and Mercurial distributed version control systems (DVCS) as well as Subversion source control.