While I prefer to use a long password to protect my computer, I like the idea of being able to use a physical key to lock it. This is precisely what Airlock does, using your iPhone. It works great.
Airlock is a system preferences pane. Double-click to install it, associate it with your iPhone or iPod touch, set the security range, and that's it. I tried with my iMac 24 and my iPhone and it worked perfectly. You can even see a radar screen showing the distance between the two devices. The moment I stepped out of the limit, the screen locked. And then, as soon as I came into the perimeter, it unlocked automagically.
There is only one problem: If you use a wireless mouse or keyboard, it won't work due to a Bluetooth bug in Mac OS X. Otherwise, this thing will make you feel like James Bond.
Without the Aston Martin. And the Martini. And the laser watch. And the Scottish accent. And the hot girls. OK, so it won't really make you feel like James Bond, but come on, get on with the program here. [Airlock via Unplggd]