Update 1/25/2007:
Sorry to my overseas readers - it has been pointed out to me by an individual in the UK that this site is only available in "limited geographies and languages".  I did not know that. :(
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So you want to play with Vista - but your'e not about to screw up your current OS / computer to give it a peak and don't want to drop cash on an OS you're not sure you'll like?

Well here's your chance to play with it  . . . for free . . . on OUR (virtual) hardware . . .
http://www.vistatestdrive.com

Looks like we have a LOT of Vista Virtual Server images being hosted on (I'm guessing) lots of physical servers . . . the VPC that I connected up to had 1GB of RAM and was pretty snappy (well - the applications launched fast - but they've got the VPC's configured to do true color so on my 3Mbps DSL connection I could watch the screen repaint).

What's the catch?  Well you have to supply an email address (doesn't seem to need to be valid . . . wait was that out loud?) and you have to install the VPC AX control on IE6 or later (sorry Firefox fanboys).  Other than that - you can try out all of Vista's cool new features for your self and there's a little lab off to the right to guide you through the process as you click around in the VPC and do stuff.