For Desktops:
Command + Option + Eject/F12 = Sleep
Command + Option + Control + Eject/F12 = Shut Down Nicely
Command + Control + Eject/F12 = Restart Nicely
(For Laptops Substitute the Power button for Eject/F12)
For Cocoa Apps:
Command + Control + D while hovering over a word pops up the definition of it.
20 Useful OS X Tips