infect your friends!
Bryan Boyer points out a smart Gmail UI doo-hickey. If you have invites available, and you're replying to someone who does not have a gmail.com address, you get an unobtrusive menu item that lets you invite that person to Gmail. And it's personalized with that person's first name. ("Invite Tekla to join Gmail.")
It's the 2004 Google equivalent of the famously viral 1996 Hotmail .sig.