Hey you guys might know this already, but Brian had no clue, so I'm passing it along. Press and hold Apple+Control+D over any word in a cocoa application (Safari, iChat, Mail etc...) and up pops a handy dandy Dictionary/Thesaurus.
You can even move your mouse over any other word on the page and it will keep throwing out definitions. Then just let go of the keys, and POOF no more dictionary.[You're fired. Get me some coffee, intern-BL]