Please list support sites and their respective endpoints/documentation here. 

• Photobucket's API now uses OAuth for all consumer authentication. They've got a number of apps connecting already, and public key signups.
• SmugMug supports OAuth on its API.
• Twitter.  FAQ.  App registration.
• Ma.gnolia. Developer Center. Register your app.
• 88 Miles. Developers Pages.
• thmbnl. Developer Wiki.
• Pownce. API Signup and Docs. API Google Group
• Google. Docs. Federated login research.
• MySpace. Documentation
• Agree2 supports OAuth. API Signup
• Brightkite API. Docs.
• Fire Eagle. Docs.
• Praized. API Signup. API Docs (draft, better soon). OSS code.
• Meetup.com. Docs.
• OpenSocial. Docs.
• Yammer. Docs.
• GetSatisfaction. Docs.
• Netflix.Docs.
• Ohloh. Docs.
• Iron Money. Docs.
• TripIt. Docs.
• Skydeck. Docs.
• Yahoo!. Docs.
• Smart.fm. Docs.
• Cliqset. Docs.
• tarpipe. Docs.
• EchoWaves. App registration
• Evernote. Docs.
• Vimeo. Docs.

Testing

• Mediamatic's OAuth Sandbox
• Google's OAuth Playground
• Termie's OAuth Echo Server
• Madgex's OAuth Echo Server