StackExchange style voting system

I'm interested in a plugin that will work similarly to StackOverflow's - http://stackoverflow.com/ - voting for forum posts.

To review:

1) I would like visitors to be able to vote up or down a WordPress forum post or answer.
2) Visitors can earn 'reputation' points based on whether their forum posts/answers are voted up or down.
3) Posters reputation scores will appear below their posts.

Is there such a plugin? or a WordPress forum that will do this?

Thanks,
Walt