Testing with IE6, IE7 and IE8 on VirtualBox. I've recently moved from using Parallels for browser testing to Sun's Open Source VirtualBox.
Here's a walkthrough on how to get a browser testing suite for free on OSX or Ubuntu. Microsoft's Virtual Hard Drives Microsoft make a [series of downloads available][1] for web developers to ensure browser compatibility. Search Results for internet explorer virtual pc (Noscope) IE NetRenderer - Browser Compatibility Check -