I've wanted to build this as a service (for fun). Basically a sight to give advice to yourself from two years in the past. Why two years? It's long enough to have some perspective while retaining enough detail to be useful. The fun bit would be that you would have to contribute your advice before you got advice from people similar to yourself.

It would be a lot of fun to try to get basic demographics out of people giving the advice and try to find out the best way to pair advice given with advice seekers. Age? Some life choice your contemplating? Gender/Orientation/Race who knows what would make advice actually useful!

Probably a terrible idea but it's fun to think about.