If you want to send a single email to multiple people, it's simple in 10.3 (I'm not sure if this worked in Jaguar or not, but if it did, I never knew about it). Just open Address Book and add, say, all your friends to a group called "Friends." Then, in Mail, when you want to send an email to all of them at once, in the "To" field, just type Friends and hit Return. All of the people in that group that have an email address in their vCard will now be added in the "To" field. This also works in the CC and BCC fields as well.
[robg adds: This works in Jaguar as well ... and I figure if one person didn't know about it, there must be more, so here it is!]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031103152744271