Win+number (1-9): Starts the application pinned to the taskbar in that position, or switches to that program.
Win+T: Focus and scroll through items on the taskbar.
Win+B: Focuses the System Tray icons
Win+P: Adjust presentation settings for your display.
Win+(+/-): Zoom in/out.
Win+G: Cycle between the Windows Gadgets on your screen.
Win+L: Lock your workstation
Win+D: Display the desktop
Win+Spacebar: Preview desktop
Win+M: Minimize all windows
Win+Shift+M: Maximize all windows
Win+E: Open Computer
Win+F: Search
Win+R: Run dialog
Win+Home: Minimize all but active window
Win+G: cycle through gadgets
Win+X = Windows Mobility Center
Win+Tab = Vista-like Alt-tab functionality
Win+Left/Right arrows: Dock the window to each side of the monitor
Win+Up arrow: Maximize the active window
Win+Down arrow: Minimize the window/Restore the window if it's maximized
Win+Right Arrow: Move window to right side
Win+Left Arrow: Move window to left side
Win+Space: All windows are made transparent so you can see the desktop
Win+Home: Minimize all windows but the current window
Shift+Win+number (1-9): Starts a new instance of the application pinned to the taskbar in that position.
Ctrl+Win+number (1-9): Cycles through open windows for the application pinned to the taskbar in that position.
Alt+Win+number (1-9): Opens the Jump List for the application pinned to the taskbar.