The best API design I have come across so far is from Zencoder [1]. Their documentation is spot on and they also have a nice little API builder. I found it interesting to study while doing designs for our own API.

[1] http://zencoder.com/api/

EDIT: After reading http://thereisnorightway.blogspot.jp/2011/02/versioning-and-... I am not so sure anymore. They still have quite an awesome set of examples though.