Lock your MAC when you move away (via Bluetooth) with AirLock
I think this is an incredible little application for your Apple products.
You'll need a Mac computer and an iPhone…
If you own either an iPhone or iPod touch along with a Mac computer then Airlock may just be the security OS X application you’ve been looking for. For only $7.77 you can have a very effective lock for your Mac computer with just your iPhone or iPod touch.