IETester is a program which allows you to test web pages in multiple versions of Internet Explorer. This program is free for personal use, or a licenses can be purchased for professional use.
The current version 0.2.3 allows for testing in IE versions 5.5 through 8 beta2. IETester allows users to simultaneously test sites with each included version of IE. Results can be viewed tab by tab, or in a split window side by side by dragging a open tab into the viewing area of the window.
The program hasn’t made it to version 1 yet so obviously there are still some quirks, but in my testing I’ve found that the CSS and HTML render the same in the IE6 component of IETester as they do in the real deal, which makes IETester the best solution I’ve found so far for anyone trying to test for IE6 in Windows Vista or later.