I noticed Safari's Preferences>Advanced>Style Sheet allows you to set your own style sheet but I didn't immediately realize this was where you put your ad blocker. I downloaded one of the more popular Mozilla ad block userContent.css stylesheets, saved it in my home directory and activated it in the prefs, and ad blocking was activated. I've been editing the file in BBEdit but so far I've had a hard time improving on it. There are some tricks for blocking Flash files but they don't seem to work in Safari. Even so, I'm switching over to Safari because the ad blocking is now good enough to satisfy me in everyday use, and bookmark management is better than any other browser I've used.
Update 6/12/03: It appears this little summary is getting linked around the web, but my link to the stylesheet example at FloppyMoose.com is undergoing a temporary outage. Now you can download the stylesheet I'm using for my own ad blocking, I'll host it on my own site for a permanent reference. If anyone can improve on this stylesheet, I'd love to hear about it, so leave a comment.
Update 4/11/04: FloppyMoose has updated his ad blocking style sheets, with improvements, so check out his site and download the latest version.