TortoiseGit is a Windows Shell Interface to Git and based on TortoiseSVN. It's open source and can fully be build with freely available software.
Gitti is a Git Client for Mac OS X designed to help users never have to touch the command line. With a highly modular and organized interface, Gitti logically separates each step in your Git workflow to maximize your efficiency.
gitti vs RabbitVCS opinions
Not runnable on the same OS (Linux)