A small thing but fairly useful and cool...
In Address Book, select a contact and navigate to Card -> Add Field in the menu. There you can add a Phonetic First and/or Last name field to the selected contact. This is quite useful for remembering how to pronounce oddly spelled or foreign names. You can also add a Birthday field and an Instant Messaging field for ICQ, Yahoo, Jabber and MSN.
[Editor's note: This may seem obvious, but when you add the fields, if you don't populate them with some data before leaving Edit mode, they won't be there when you return to the contact. And as far as I can tell, there's no way to add the field to all contacts at one time.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20021118062715761