Historical versions can still be found at http://ozlabs.org/~paulus/gitk/
See the main git repository for current versions.
The man page: http://www.kernel.org/pub/software/scm/git/docs/gitk.html
Displays changes in a repository or a selected set of commits. This includes visualizing the commit graph, showing information related to each commit, and the files in the trees of each revision.
Historically, gitk was the first repository browser. It's written in Tcl/Tk and started off in a separate repository but was later merged into the main git repository.
