Screen readers.
ZoomText 9+, JAWS 7.1+, NVDA, VoiceOver, FireVox, Orca. All differ, none are
perfect.
JavaScript
libraries: Many popular libraries (JQuery UI, YUI, Dojo, ExtJS, GWT) )are
in the process of adding support for ARIA. Great resource is http://www.paciellogroup.com/blog/?p=313.