The majority of the time when we look at AJAX we look at making websites more responsive, prettier, faster. Rarely do we (or at least I) think about accessibility features for people that are visually impaired, or deaf or otherwise less able to read my, usually, small print on a website.
Chris Blouch has released a JavaScript library which focusses on making the web 2.0 generation of website more accessible. He has posted a slidedeck from his session at the Ajax Experience Boston (October 26, 2007) online, and the library, including documentation, can be found here.
I particularly found his statistics slide an eye-opener...