Git Extensions is a toolkit to make working with Git under Windows more intuitive. The shell extension will intergrate in Windows Explorer and presents a nice context menu on files.
Online manual: https://git-extensions-documentation.readthedocs.org/en/latest/
The closest I've found so far to "TortoiseHg for Git" (which is by far the best GUI client for any DVCS I've seen). Which is ironic, given that there is a "TortoiseGit" (which I am not very fond of). Compared to TortoiseHg, Git Extensions could use some polish though to make the GUI more ergonomic to get on par with TortoiseHg (it's mostly minor things like borders and colors - yes, such things do matter), but overall I am quite fond of it.