We are all creatures of habit - so how about this one — A community of people who share their habits and help their friends drop bad ones and pick up new ones. You could be cheesy and call it “Habitr” 
I used to work with a programmer that brushed and flossed almost religiously after lunch. I always said to myself “I want to pick up that habit” but I never really did until I started working from home. I’ve identified some bad habits that I want to shake and I’m sure I’ve got a bunch more that I’m not even conscious about.
So how about a site where you can tag what good and habits you see in yourself. What if your friends could tag habits on you that you don’t even notice. Some fancy sql queries later you can discover helpful habits/techniques you didn’t know about. What if there was also some kind of wiki/discussion system where people could talk about how they dropped some habits (i.e. smoking) and how they picked up good ones.
With the Facebook API you could take advantage of its massive social network - this tool could spread pretty easily. Revenue could come from ads or linking to self-help books on Amazon.
Good idea? Bad idea? I guess my first habit would be “blogs about neat but intangible website ideas” 
