A really nice explanation by Todd Kloots of Twitter’s use of progressive enhancement with Ajax and the HTML5 History API. There’s even a shout for Hijax in there.
Remy teaches non-techies how to use jQuery in a responsible way.
The Spry framework from Adobe looks like it could be worth further investigation. I certainly like the underlying philosophy: lightweight, standards-based, and declarative.
Cameron shares his thoughts on Ajax, Hijax, libraries and having fun.
Garret gives an excellent, excellent round-up of the factors involved in the behaviour layer of front-end architecture (that's 'building websites' to you and me).
The slides of the Hijax talk from the Ajax Developer's Day at XTech 2006 in Amsterdam.