I remember using OWA (Outlook Web Access) before that.. but you are right this is the one that stands out..... soon after this Digg was created and the onslaught of all the web 2.0 sites like delicious, flickr etc etc
After all XMLHttpRequest was a non-standard addition to the web API by Microsoft, specifically added for Outlook AFAIK. Firefox copied it though, so GMail could use it without sacrificing browser agnosticism.