Rails 3. Building an oauth2 provider
Check out this gem https://github.com/applicake/doorkeeper
It is for Rails 3, the development it's early stages though.
There's also an example app that you take a look and see how the API is done.
https://github.com/intridea/oauth2 is the canonical gem right now for OAuth2 dev. If you are looking for a more complete solution, instead of rolling your own provider code, check out:
But I suggest messing around with the oauth2 gem if you aren't very familiar with the flow so that you can learn it better.
I've opensourced an OAuth2 server implementation yesterday.
It's well documented and there is a dashboard to control accesses. Right now I'm searching for somebody who wants to build an engine starting from it, or something cool on top of rack. That's why it is open-source.