The version control par excellence in design is Subversion, but Git is hitting very hard in recent times thanks to the support it is receiving from many developers and the popularity of its creator, the great Linus Torvalds.
Gitbox is a graphical interface for Git that opts for simplicity as its strong point, since in a single window we can have access to everything necessary for the Git control system. Further, the toolbar is quite simple and that makes it easy to submit new changes quickly and transparently.
Has FileMerge or Kaleidoscope integration to see if two files differ, something that is appreciated because it can save a lot of time in different cases.
The application is, of course, free.
Download | gitbox