This one came across my Inbox last week and I didn't get around to blogging it until now. The folks that make the DebugBar (free for private use only) and Companion.JS (free) addons for Internet Explorer have released a new (also free) tool called IETester.
It works in both Windows XP and Windows Vista when you have IE7 installed as the default browser and allows you to test in IE5.5 and IE6 (I'm not sure if IE8 Beta 1 has any negative effects on it though).
It's only an alpha release but from the quick testing I've done it seems to work quite well. The only immediate issue I had was that it assumed Flash support was available but it was unable to render Flash content on any of the sites I tried.
So if you aren't a fan of running a virtual machine all day long this could be a promising option :)