Another one for the you-don't-need-this-but-if-you-need-it-you-need-it-bad department: Winspector.

If you need to look at your Windows desktop from underneath, seeing all the gruesome details of all the windows, dialogs and controls, and all the messages being sent around, this is for you. If you don't have access to Microsoft's Spy++, or even if you do, it's worth a look. Useful, for example, when using winGuiAuto.