I found Little Dutch Moose (a shareware utility) on Versiontracker, downloaded it and installed it. If you're running apache on OS X, it adds a system preference pane that automatically adds IPs to the built-in firewall if the IP asks for certain files or directories (system32, WINNT,etc) which are characteristic of Windows worm viruses - things like "Nimda" and "Code Red", for example. By blocking IPs from these infected hosts, your bandwidth is saved for actually serving your pages.
Nice interface and logs. In addition to shutting out some bandwidth hogs, it makes my referrer log and error log cleaner.
Anyway, thought you should know.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20020206090906740