One example would be that I saw someone using Github as their blog.
https://github.com/technoweenie/madrox - Distributed Twitter implementation in Git.
https://github.com/schacon/ticgit - Git-based distributed ticketing system.
https://github.com/progit/progit - Pro Git book using GitHub to facilitate translations into 25 languages.
http://code.google.com/p/gource/ - Visualize the activity on a git repo (OpenGL).
http://kitenet.net/~joey/blog/entry/databranches/ "databranches: using git as a database"
http://git-annex.branchable.com/ "git-annex is a git subcommand that allows tracking large files in git -- without the bother of feeding their contents into the git repository!"
sparkleshare is a dropbox-like system that uses git as a back end (Mac OSX and Linux only for now)