Alt-F4 helps you ignore posters you don't like.
I wonder how many people will try this one.
And Mandalee mentioned the rest of the ones I know for PC.
Most of it is the same on a MAC
Command C Copy
Command X Cut
Command V Paste
Command D Show/Hide Dock
Command - Tab Switch application
Tab - Highlight next item
Option-Command - esc Force Quit
Control - Eject Restart, Sleep, Shutdown dialog box
Control-Command - Eject Quit all applications and restart
Option-Command - Eject or Option-Command-Power Sleep
Command - Cycle through windows in application or Finder (if more than one window is open)
Function-Delete (PowerBook, iBook only) - Forward Delete (delete the character to the right of your cursor)