If you use emacs, magit apparently also has this functionality (I haven't gotten magit to work yet). I personally use http://porkrind.org/commit-patch/ via its Emacs integration, which has the benefit of working for VCSes other than git (though I personally find that becoming more and more rare as time goes by). Commit patch also has the command line tool "commit-partial" in case you don't use Emacs.
Disclaimer: A friend and I wrote commit-patch.