Did you know about the built-in dictionary? This is in OS X since Tiger: It's a weird key combo, but it's sooooo cool: In Mail or TextEdit or Safari -- any good Mac app (which excludes, par exemple, MS Office 2004 but includes Office 2008) -- put your cursor over any word, and hold down ctrl-command-D. Cool, huh? (The "command" key is now called that on newer keyboards; it used to be the key with an apple and a squiggly thing.)
Now move your cursor over other words while holding down those keys. Notice that there's a thesaurus in the drop-down that says "Dictionary." Now tell me that's not friggin' sweet.