I have to admit until recently I’ve been using virtual machines for testing rendering in different versions of IE – just came across IETester which lets you open new tabs running the rendering engine from different versions. You probably know all about this already, but it was new to me. Last time I looked into this it was still a major headache.
I’d really recommend taking a look if you’re doing any client-side dev work.
Update: There isn’t a huge amount of information about exactly how this is implemented and whether you can solely rely on this for testing rendering + javascript of a major application. The closest I could find was this from the author on a forum… ‘I am hooking win32 functions to override standard calls and launch custom versions of IE instead of standard version.’ Everything I’ve tried so far has been accurate, but I think better documentation would really help adoption.
There may be info I haven’t found in the forums.

