A service I'd pay for: Mailing list management that doesn't suck, and isn't Google Groups. Something that can be backed up, and has a web interface. Mailman is still king in this niche, and I'm not entirely sure why.
This is a good idea, surely someone has already made a good mailing list tool?
Zed Shaw's http://lamsonproject.org/ looked interesting, last time I looked at it. (If you forgive it for pretending all the world is UTF-8, which it emphatically isn't.)
There is basecamp's breeze. For some reasons, it's shutting down August though. basecamp.com/breeze