In Leopard's Mail, when typing in the recipient of an email message, Leopard will search the Address Book and show matching addresses by the person's Nickname field. This is particularly useful when you have contacts with names in foreign character sets (like Japanese), and keep a nickname in roman characters, but of general utility nonetheless.

For example, suppose the person's name is "小野 洋子" and the nickname is set to Yoko. By typing only yoko in the To: field of a message, Mail will autocomplete the email address.

[robg adds: This tip makes this one irrelevant, thankfully. I had created a number of one-user groups that have now been replaced by nicknames. You may have to use the Card » Add Field menu item to add Nickname if you've not used it before.]