Hold down the  Windows key and press:
E to open Windows Explorer
R to open the Run dialog box
F to open the Find dialog box
M to minimize all open windows (Shift-Windows-M to undo minimize all)
Tab to cycle through the Taskbar buttons
Break to open the System Properties dialog box

Using Explorer
Press F4 to drop the "folder selection menu" in Explorer
Press F6 to switch between left and right panes
Press Num Lock + Asterisk (on numeric keypad) to expand all subfolders of the selected folder
Press Num Lock + Plus sign (on numeric keypad) to expand the selected folder
Press Num Lock + Minus sign (on numeric keypad) to collapse the selected folder
Press Right Arrow to expand current selection if ifs collapsed
Press Left Arrow to collapse current selection if its expanded

Opening Files With Different Programs
Hold down the shift key while right-clicking on the file in Explorer; the 'open with' dialog will appear, and you can choose an alternate program to open the file with.