Edit for 2012: Try Google OAuth 2 playground at https://code.google.com/oauthplayground for a practical demo. It will allow you to see working requests.
There aren't any good, generic OAuth 2 tutorials. Obviously the IETF spec, like any engineering spec, is designed to be specific and verbose rather than short and simple.
Additionally many other oAuth 2 documents are diffs of oAuth 1, which is a horrible way to try and learn something.
I find Google's own (Google-specific) oAuth 2 documents relatively good to get an understanding of how it practically works - what's sent, in what request, to whom:
https://developers.google.com/oauthplayground/
