I am using Atlassian SourceTreeAtlassian SourceTree 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:
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.