Git on Windows: How do you set up a mergetool?

I’ve tried msysGit and Git on Cygwin. Both work just fine in and of themselves and both run gitk and git-gui perfectly.

Now how the heck do I configure a mergetool? (Vimdiff works on Cygwin, but preferably I would like something a little more user-friendly for some of our Windows-loving coworkers.)