Command-Q
Quit any program
Command-W
Close whatever active window you are using,
Option-Command-W
Close all the windows in the currently active app
Command-T
Open new tab
Command-Tab
Switch Apps
Command-X, Command-C, Command-V
Cut, Copy, and Paste
Command-F
Search
Command-Shift-3, Command-Shift-4
Snapshots
Command-Shift-A, Command-Shift-U, Command-Shift-D, Command-Shift-H
Navigation in Finder
Command-Option-Esc
Force Quit
Command-H, Command-Option-H
Quickly Hide Windows