Here is another AtomAPI implementation and this one has some interesting extensions. There are additions for creating new users and creating new groups. Particularly interesting to me is the use of the HTTP OPTIONS method supported on the Post, Edit and Feed URIs.
SixApart has their own extension in their implementation. Their extensions include an UploadURI for uploading any file, and a CategoryURI used to query the list of allowed categories an entry can be classified under.
It's heartening to see the innovation in these implementations. Keep it coming.