AIM

I like the AOL instant messenger app, but it's not notifying me of new messages unless I have it open. In the keynote, Jobs said they were giving developers a workaround notification system for exactly this kind of scenario. The application even has a preference setting for whether or not it quits when you close it. I don't really know how the notifications are pushed, but maybe the server that's supposed to do that is being overloaded right now, like MobileMe.

Update: I guess I overlooked this before, but according to arstechnica, the push notification support isn't scheduled to be released to developers until September.