To my knowledge XMLHttpRequest was introduced within IE 5.0 when it was released with NT or Win 2K (now we are talking about IE7 with Windows Vista).
The companies which have released some sort of AJAX framework are:
1. Our big brother "Microsofts" framework for AJAX is called Atlas and you can read about it here.
2. Google's version is called "Google-ajaxslt". One can download their framework here.
3. Check out Project jMaki from SUN.
4. Yahoo's version is "Yahoo! User Interface Library"
All the above mentioned companies where competing directly/indirectly with each other for quite sometime now. But the surprise entry (Atleast for me) is Adobe.
Adobe has released a AJAX framework by name SPRY.
Offlate almost all major technology companies are into AJAX hmmm let the technology WAR on AJAX begin :)