Personally I was looking for a way to do this back in 2000 when we were all using IE4.
It took nearly three years for first implementation to appear back in late 2003 releases in IE5.
"This" refers to being able to update a part of the screen without whole thing refreshing.
That means avoiding a whole lot more traffic on the wire, server processing a load of events such as the page load event.
.
At the time, 2003 such functionality was wrapped up as an ActiveX component and was limited in it's use.
There's a large team at Microsoft responsible for this very important API / functionality; I don't know who "cracked it" but for now but lets just say there was a great team behind it.
This is of highly significant piece of technology developed by Microsoft for Web Applications was
officially release appeared on January 27th 2007, a month later the WC3 draft (below) has been released.
WC3 offical Draft Feb 27th 2007
We find ourselves flooded with Ajax products now which is cool but the toolkit is free;
from Microsoft here.
A word of
caution though, Ajax is great for Web Applications, "regular" web sites will find it can be can mess-up a few things, namely Search Engine indexing of your pages.
Always test and test again, for example Googles robots that spider and subsequently index you pages can only see hypertext on your pages, so if you wrap e.g. a grid of hyperlinks inside an update panel (JavaScript) then you will find your links are no longer indexed.