A few months ago I bought a copy of VMware, and now I can't imagine how I tested my software without it. Previously, I'd create separate partitions for each version of Windows, which required rebooting into the desired OS in order to test – a tedious way to do things.
As this screenshot illustrates, testing is a breeze now. I can quickly start up Windows 95, 98, ME, 2000, XP, 2003 and even Longhorn without ever rebooting.
This also simplifies testing my site on different versions of Internet Explorer (although this has become easier anyway thanks to this handy tip). [Nick Bradbury]