I had this idea not too long ago. An assistant as a service, that would be accessible on IM and would do increasingly more things, starting with handling the scheduling of appointments between two people completely automatically (it would email them and ask them if they're available, of course).

I made a small prototype, but it was way too limited, as NLP is a huge problem. Even a simple TODO list/alerts/meeting scheduling app with calendar integration was very, very complicated.

I'm pretty sure lots of people would pay for it, though.