Microsoft unveiled a preview release of Internet Explorer 9 today during their Mix 10 keynote. It aims to support all the latest web standards. You can grab the preview from http://www.ietestdrive.com, unless you happen to run Windows XP. It seems IE9 throws XP under the bus, so those users will have to stick with something like Chrome if they want to be on the bleeding edge.
This is mostly for developers to try out, so some basic features are missing. Namely, a Back button and some security. That being said, you may not want to do anything serious with this release like manage your bank account or buy stocks. Microsoft has stated that they are committed to a new update every 8 weeks and will keep their finger on the pulse of the community using Microsoft Connect.
An interesting thing to note is that one of Microsofts goals for this next release of Internet Explorer is to reduce developer headaches by improving code compatibility across all browsers. A lofty goal for the browser has developers and designers around the world banging their heads on their desks most of the time. Some of their more realistic goals include making the browser more responsive and maintaining compatibility with existing websites.
MS named no actual Beta release date and seem to be intent on taking things slow for now. They are still gradually increasing support for stuff like HTML5 and are interested in raising their ACID3 score. According to officials, the build out today is at a 55.