World Wide Webster

I've done a lot of trash talk about Mac OS X Tiger but I still resolutely insist that it kicks ass, and one of the reasons why is the operating system's new Cocoa-based dynamic dictionary and thesaurus lookups.

You can invoke a dictionary lookup within any Cocoa application - one of the best indicators of those is the presence of the notorius font pane by holding down command-D and simply hovering over any given word. (Via Subtraction)