We use http://readme.io and they are incredibly awesome. I can't give them a stronger or more positive recommendation.
On the bottom of the page it says "Powered by readme" and links here: https://readme.io/
Hey, I'm one of the ReadMe founders. Stripe was certainly an inspiration for us. I'm so psyched to have SEED building their API using our documentation tool—nice job SEED!
We couldn't have done it without y'all, thank you!