This USB Wireless PC Lock from Think Geek is actually a pretty innovative security system. It consists of two pieces, a USB dongle which attaches to your PC and a battery-powered keychain transmitter. As soon as the transmitter is more than 2 meters away from the PC, the PC automatically locks the PC. Once you get back within 2 meters — unlocked. Yes, there is a manual override in case your transmitter runs out of batteris or you lose the device.
The software is currently only compatible with Windows and does not work with dual screen computers.
Available from Think Geek for $29.99.
Wireless PC Lock