Possible Duplicate:
Running Internet Explorer 6, Internet Explorer 7, and Internet Explorer 8 on the same machine
Is there a way to have Internet Explorer 8 and Internet Explorer 6 side by side without virtualizing?
I used Multiple IEs which works fine with Internet Explorer 7, but since I installed Internet Explorer 8 beta 2, Internet Explorer 6 started behaving oddly (that is, more than usual).
There's also IE7 standalone
Virtualization is the easiest way to achieve this. It has a higher overhead, but since IE has so many hooks into the OS trying to install multiple versions of it is doomed to confusion and failure.
One more multiple, standalone IE option: IE Collection.
I use a utility called "Sandboxie" (free for personal use, $29 for commercial) to provide application sandboxing. One useful side-effect of this is that you can install applications (even OS-modifying ones such as IE) into the sandbox, and the parent OS is completely unaware (allowing you to have different versions of the parent OS's IE and the sandboxed IE - and both running simultaneously). The two scenarios I've used so far:
Caveats:
Try this: http://www.my-debugbar.com/wiki/IETester/HomePage
LE: This isn't really fully compatible yet, there are a few minor issues, like it crashes on JavaScript pop-ups, but I've found it quite reliable during development. At the end of everything, I just tested the web application against a real IE6 to make sure everything's fine.
What I do is use VMware with other OS with IE6.
Not perfect, but it helps.