Git has a couple of fairly popular Graphial User Interfaces that can read and/or maniplulate Git repositories.
Git comes with two major GUI programs written in Tcl/Tk. Gitk is a repository browser and commit history visualization tool.
git gui is a tool that helps you visualize the index operations, like add, remove and commit. It won't do everything you can do on the command line, but for many of the basic operations, it's pretty good.
For Mac users, there are GitX and GitNub
For Linux or other Qt users, there is QGit