Don't know how well known this is, figured I'd post it anyway.
Basically, it prints a picture (render) of what your page looks like in various versions of Internet Explorer. All you do is simply input the addy to the page you want to render. I always double-check my layouts personally on different operating systems, using different browsers, but this is a great tool to do quick compatibility checks on the fly. A friend of mine sent me this, and I've used it a lot since.