You can also add a fingerprint scanner relatively easily.

Fingerprint scanner (images are CC BY-NC-SA 3.0)

The scanner once again plugs into a USB port. It does require a fair amount of software to control, but most of the actual reading and whatnot is handled by the fingerprint module. What you have to do is add prints to a database in a setup mode and scan for them on an operate mode. This device requires a USB-to-TTL adapter, in order to connect to the BeagleBone in the same manner as the other devices. SparkFun sells several versions of these adapters.