Web developers have been using the very handy IE standalone for a while now. It gives access to IE versions dating back to IE3. While a handy tool to check backwards compatibility there are some limits and drawbacks.
Now Microsoft, yes, Microsoft, have come to the party and released a virtual hard drive that comes with a licensed copy of Windows XP SP2 and IE 6.0 installed. While targeted for the cross over time as people update from IE 6 to IE 7 it’s an incredibly handy (yet large) download. All you need is the, also free, Microsoft Virtual PC and you can have fully functioning IE 6 and 7 on the same PC. Hopefully further down the track they may release the 5.0 versions as an alarming number of people are still using these dinosaurs.