<3 doorkeeper. Using it to make one of our open source apps - http://github.com/isotope11/xrono - an oauth provider to support an oauth sso flow inside of apps we've built for internal use regularly. It took an hour one night to build the provider, and maybe an hour to build the omniauth provider gem and integrate it into an app.

P.S. - it also took like six hours of banging my head against a wall for unrelated oauth-related problems, but not DK's fault.