Entropy

LH author Adam Pash has a great article on BlueLock, a bluetooth based tool for Windows which will lock your desktop whenever you walk out of range. Essentially this allows you to automatically have your computer lock whenever you leave.

The usefulness will depend on the strength of your Bluetooth devices, since it’s not really going to do its job if you’ve got a strong signal and receiver that keeps a connection from across the office

If you’re a Mac user, LH (of course) has an older guide on doing  this and much more based on Bluetooth proximity.