Keyboard vs. Mouse - Today's Winner: The Keyboard (poor little mouse!)
Windows XP Home and Professional (Some work on other versions of Windows too!)

The Windows key brings up the Start menu,  but it also does quite a lot more too!:

Winkey-D press it to view your desktop, press it again to go back to where you were :-)

Winkey-M minimizes all windows.

Winkey-E opens the Windows Explorer.

Winkey-L locks your system until you enter your password—or lets you switch active users (if you have Fast User Switching enabled).

Winkey-R brings up the Run dialog.

Winkey-Pause/Break key brings up the System Properties dialog.

Winkey-U opens the Windows Utility Manager, (accessibility program options).

Winkey-S, in Microsoft Word 2002 or later, invokes Windows' text-to-speech engine, which will read either highlighted text or everything from the cursor on. (It's very annoying too.)

Here's some other useful keyboard tips -