IE9 preview 3: better performance, HTML5 video and canvas, more awesome demos
That said, it's clear that Microsoft is serious about delivering a competitive browser. IE9 preview 3 has turned in better SunSpider and Acid3 results, and its hardware acceleration features really shine. The new release also features HTML5 audio and video support, as well as hardware acceleration for the canvas element. Support for Web fonts is now baked in as well.
Running the fledgling browser side-by-side with Google Chrome 6 I noticed very little difference on sites like Gmail, Facebook, and Google Reader... And good luck getting another browser to run Microsoft's standards-based demos as well as they run in IE9. They're pretty brutal without hardware acceleration (with the exception of Opera 10.60, which actually seems to perform better than Chrome and Firefox -- Sebastian has a video on the way). The SunSpider result has improved again, too, and is now roughly half what the first preview posted -- an impressive gain.
For full details about the new release, check out the official blog post from Microsoft.