IE 9 was released last month. It requires Windows Vista or 7.
It looks like it is
the most standards compliant web browser, via an extensive standards test suit which I have great respect for, which other browsers have had a disappointing response to.
HTML 5 conformance testing from the HTML standards folks.IE versions prior to 9 have
not worked on my web site for a couple years, due to requiring support for a standard from 2000 which IE lacked (XML content type). As soon as I heard IE 9 added support, I did my best to ensure that it works on my site.