Launchy is a simple, light-weight, open-source application that allows you to open applications by searching for them. Once you’ve installed the app, hit ALT+SPACE and the little launchy window appears allowing you to search for the application you want to open. Mac users have had the same thing for a long time with Quicksilver.
Apart from just launching applications much quicker than finding them in the quick start or god-forbid the Start Menu > Applications (which has, admittedly, improved a lot in Vista), it is also very useful as a simple calculator. Just launch Launchy (ALT+SPACE) and start typing in what you need to calculate.
Launchy also has a bunch of skins and plugins but I haven’t used any of them. If you know of any really useful ones, let me know in the comments.