command-control-D
Invoke Tiger's dictionary in any Cocoa app by hovering over the word with this key combination. So handy. Visit site
For those that don't know, 'Command' is also known as the 'Apple Key' on your keyboard. A Cocoa app is, as far as I understand it, something that has been designed for OS X. If you're a new mac user, then most of your apps should be 'cocoa'ized.
