(This is not exactly a "Windows" standalone Git GUI, but still quite advanced)
Considering that since then, Eclipse is in the process of mirroring all its projects in Git repositories, EGit (based on JGit) have made some progress and are part of the Eclipse ecosystem, and is on par with its Mercurial counterpart.
You can contribute to EGit there.
This Tutorial is quite complete.
alt text http://www.vogella.de/articles/EGit/images/github60.gif