Windows*
Close current application ALT-F4
Switch tasks ALT-TAB (2D)
Windows-TAB (3D)
Lock your computer Windows-L
Display task manager CTRL-SHIFT-Esc
Run Windows-R
Launch a program in Vista Windows and start typing
Show desktop Windows-D (toggles)
Windows-M (no toggle)
Go to System Tray (icons beside the clock) Windows-B
Launch Explorer Windows-E
Launch application on Quick Launch bar Windows-1 to Windows-0
Cycle through task bar Windows-T - space selects
* Most work on Windows XP/Server 2003 and above, though some were added in Vista. So your mileage may vary.
FireFox
Open a new tab CTRL-T
Close current tab CTRL-F4
Switch tabs CTRL-TAB (forward)
CTRL-SHIFT-TAB (backward)
Re-open last closed tab CTRL-SHIFT-T
Go to search box CTRL-K
Go to address bar CTRL-L
Find CTRL-F
Go back ALT-left arrow
Go forward ALT-right arrow
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