I happened across Xobni today (Inbox, backwards … ain’t that cute!) It’s an interesting idea and it looks like they’ve been around for quite a while in one form or another (their weblog goes back to April 17 of 2006). It’s still an invitation-only beta but it looks like invitations aren’t all that difficult to get. There’s a little discussion of them in EmailDiscussions.com.
No, I haven’t tried it and I’m not sure I’ll have the time soon so if anyone has any experiences with them, please let me know.