This one is pretty cool.

double click [with three fingers] on any word
it will highlight and the inline dictionary will pop up with an explanation of that word.

If the dictionary finds a match in any of your sources (configured form the dictionary app’s preferences), then it will show an entry from each that match.

its a great addition that so far Ive found works in all the apple built in apps, eg safari, mail.app, textedit, etc