On October 31st 2007 as reported in many places, there's a new OS X malware in the wild, first reported by Intego, who named it OSX.RSPlug.A

How to remove the malware:
    1.    In the Finder, navigate to /Library -> Internet Plug-Ins, and delete the file named plugins.settings. Empty the trash. This deletes the tool that sets the rogue DNS Server information.
    2.    In Terminal, type sudo crontab -r and provide your admin password when asked. This deletes the root cron job that checks the DNS Server settings. You can prove it worked by typing sudo crontab -l; you should see the message crontab: no crontab for root.
    3.    Open your Network System Preferences panel, go to the DNS Server box, and copy the entries you can see to a Stickies note, TextEdit document, or memorize them. Now retype those same values in the box, then click Apply.
    4.    Reboot your Mac.

The only people who should be infected are those who have broken the number one rule of internet computing: don't download and install programs [especially those that are (a) package installers that (b) request your admin password] from untrusted sources.