From what I can tell IE 5.0+, Opera 8.02, and at least Gecko 1.0.2/20021216 support AJAX. I'm not sure about OSX browsers though I'd imagine they all currently support AJAX in their latest versions (OmniWeb, WebKit, Opera, and Gecko). Also I'm curious as to how far back KHTML (Konqueror) supports AJAX.

A side note, I've noticed that Gecko 1.7.13 and older do not correctly load AJAX include files while my page is served as application/xhtml+xml (though I'm just mentioning that in case anyone encounters that or anyone can make a meaningful (not don't use that media type) suggestion or comment). However from that early version I just mentioned and newer all load AJAX fine while the page is served as text/html.

- John