SSH to your stolen laptop
A holiday present for Hacker News (and you too!): tund, a daemon to automatically maintain reverse SSH tunnels. If your laptop gets stolen, and the thief connects it to the internet, tund will reach out and establish an SSH tunnel from somewhere.com to itself. That means you can log into it, even if it's behind a firewall or NAT.
I also use it to log into my home computer from work.
Update (15:36 PST): Hey, I made frontpage!