WebFX was my first encounter of XmlHttpRequest.
This to me was the first memorable intance of RIA as I was blown away by how complex it all seemed.
I'm still a big fan of Erik & Emil's work, and check out his blog from time to time. I think Erik & Emil are now over at Google no doubt doing some amazing things inside the AJAX space. I interviewd Erik & Emil over at my other blog called MossyBlog.com - (I'm yet to figure out what to do in terms of merging the two) and still firmly believe these two were the source of inspiration behind AJAX (GMAPS + GMAIL) - but I can't substantiate that claim with facts/timelines.