Windows: If you enjoy tweaking your OS right down to the little things like the start button icon, Windows 7 Start Button Changer makes it simple to swap the stock icon set for a new one.
Windows 7 Start Button Changer (W7SBC) is a stand-alone and portable application. Unzip it, run the application, and click on "Select & Change Start Button" to swap your icons. W7SBC comes with 10 alternative sets for you to try out. If you're not happy with any of the included sets you can either try your hand at making your own or you can head over to Deviant Art and check out the huge selection of start button icons in the customisations category.
W7SBC is freeware, Windows only. Have a favourite application for tweaking the Windows interface? Let's hear about it in the comments.