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.
jjmarcus tech solutions 
Holy carp! That’s friggin’ sweet.
Is that documented anywhere, or are people just supposed to discover it somehow?
It’s buried in Help in Dictionary.app. Like everything else (on any OS, I suppose), one has to be either a) optimistically trawling for a specific feature or b) scouring the various sites like macosxhints.com on some kind of regular basis.
BTW, I made a big boo-boo typo: It’s ctrl-COMMAND-D, not ctrl-option-D.
Alleluiah! Wait did I spell that write? Let me ctrl-COMMAND-D it. Oops. No it’s wrong…