I can't believe no one has mentioned
xGestures a $5 app that adds mouse gestures to the entire OS (customizable per Application). People drop their jaw when they see me working.
I have them set to go off with my middle mouse button (and draw to the screen so I can double check I'm doing the right one).
My favorite gestures:
Up - Expose All Windows
Up Left - Expose Desktop
Down - Close Tab
Down Right - Minimize to Dock (my dock is on the right side of the screen)
Down Left - Close Window
Down Left Right - Quit Application
Web Browser
Left - Previous Tab
Right - Forward Tab
xCode
Right - Build and Run
Down - Build and Analyze
Left Right - Switch between header and implementation
Left - Code Pilot (useful xCode addin)
Extra tip: having all these actions available as gestures freed up my 4th and 5th mouse buttons which are now copy and paste which is incredibly useful.