A simple hint -- double tap a word with 3 fingers to get a pop-over with dictionary and thesaurus definition! Works in most apps.
[crarko adds: When I tried this (in Safari) I got a pop-up menu, that gave me the option to Look Up the word, as well as other options. I also got the spinning beachball of death when I tried it in TextEdit. 10.7.1, anyone?]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110720151624510