App: USB Lock
Developer: Chris Vallis
Genres: Productivity, Utilities

As seen on Lifehacker and Addictive Tips

Lock and unlock your screen with a usb storage  device.  

Features:
Screen turns black or displays selected image when locked and optional password text box appears when key pressed or mouse clicked. 

Use global hot key to lock the screen. 

Does not sleep or shutdown running applications. Leave applications running while the system is protected. 

Use with a password for Two-Factor authentication.

Two-factor authentication requires the use of two authentication factors.  The two factors are something the user knows and something the user has. For example a USB device and a password.

Coming Soon: 
Idle timer to lock the screen when system inactive.

Release notes:
multiple screen support