If you're talking about formats other than DVD/AC3/DTS, then:
http://www.windowsmedia.com/9Series/DemoCenter/AudioQuality.asp?page=6&lookup=AudioQualityand
http://www.windowsmedia.com/9Series/DemoCenter/VideoQuality.asp?page=6&lookup=VideoQualityThis link has audio and video samples in Windows Media 9 format, which are multichannel. Unfortunately MC9 only plays them in stereo at this time, and I haven't heard of a plan to change that.
If you keep an eye on windowsmedia.com, you'll find other treasures but they seem to pop up and then disappear. Pink Floyd's Money in 5.1 WMA was up several weeks back, and sounds absolutely amazing. Again, you must play it in WMP9 to get the full effect.
On an aside, at least MC9 will play them, if in stereo - ZoomPlayer and (I assume) many others can't play files such as Money because they don't support Digital Rights Management. MC9 will play these files back as long as you first play them in WMP9 to obtain a DRM license. It would be a complete package, were it not for the lack of Windows Media multichannel support.
Finally, a while back there was an option to order the "Do Amazing Things" DVD from Microsoft, for no more than shipping costs. Among other things it includes a bunch of BMW commercials in high definition video, with killer audio tracks.
One caveat to this whole thing: if you are looking for surround sound Windows Media format material, you must output via analogue to your receiver. You cannot use a digital cable, as this limits you to either pre-encoded Dolby Digital or DTS, or 2-channel PCM sound. The only 'workaround' is an nForce2 motherboard which can transcode multichannel Windows Media audio into Dolby Digital on the fly; however you lose audio quality in the process. The best solution is a 24/192 capable multichannel analogue card such as an M-Audio Revolution, Audigy2, or higher-end card like a Delta 1010, etc.
I have a HTPC with Audigy2 going into a 6.1 receiver using analogue cables, and it sounds wonderful.
Hope this helps!
-Doug