eWeek reports in this article of a security exposure in Mozilla's Mozilla, Firefox, and Thunderbird clients wherein a malicious user can cause arbitrary code execution by using the shell: protocol. You can download Mozilla 1.7.1, Firefox 0.9.2, and Thunderbird 0.7.2.
X11 forwarding is a powerful and helpful feature of SSH but, as pointed out in this article from Hacking Linux Exposed, it can be dangerous if you're not careful. It's no great secret -- pay attention to your file and directory permissions!
Microsoft has some additional information available about the most recent security exposure here. It tells you how to determine if you've been infected.
I'm a little behind on this as it's dated June 8 but better late than never. Here's a link to PC Magazine's 2004 Utility Guide. Backup programs, defraggers, anti-virus, file managers, etc.