I am using [Atlassian SourceTree][1] and like it a lot.

Here's the drill:

 - Free (not open-source thought AFAIK)
 - Feature rich - Almost all the features of Git is there (not of GitHub, though, e.g. I didn't find a way to rebase a GitHub fork. It's doable using ordinary Git commands - adding remote etc, but not out of the box)
 - Supports GitFlow
 - Nice UI:

   [![screenshot][2]][2]

NOTE - This screenshot is from a much older version. The UI of the newer version is simpler.

Bottom line - I find it almost perfect and use it for all the needs not covered inside my IDE.

  [1]: http://www.sourcetreeapp.com/
  [2]: https://i.sstatic.net/TSRKo.png