The GitHub App for Mac[1] and Windows[2] as well as msysgit[3] (and probably others) have this feature as well. I always resort to these GUIs when only commiting parts of a file, because it's a very visual process and I believe a GUI is the right tool in this case.

[1] http://mac.github.com/

[2] http://windows.github.com/

[3] http://msysgit.github.io/