Alex (my first manager at Microsoft) write a great article on the birth of XMLHTTP (the core technology that underlies AJAX). Definitely a great read.
Though Alex was kind enough to mention my involvement, I have to say that XMLHTTP was pretty much done before I got there. My role on the Exchange team was to shepherd the Exchange web development platform -- basically WebDAV on the server -- to shipping (granted, that took about 18 months), along with other PMs like Lisa Dusseault (who is Applications Area Director at the IETF these days) and Karim Batthish (the only one of us still working on Exchange).
WebDAV and XMLHTTP formed the core of the Exchange app development platform. One of things I did as the eventual PM for WebDAV was encouraging developers to build AJAX applications on top of Exchange long before the AJAX concept was well-understood. Needless to say, a lot of people didn't get it ("you can change the HTML in the page after it's written? Huh?"). Lots of fun.
I'm about to get all nostalgic, so I'll stop now. :)
(found via Dare -- I sort of lost track of Alex after he left Microsoft. Didn't even know he had a blog)