KeySweeper is a stealthy Arduino-based device, camouflaged as a functioning USB wall charger, that wirelessly and passively sniffs, decrypts, logs and reports back all keystrokes from any Microsoft wireless keyboards (which use a proprietary 2.4GHz RF protocol) in the area.

Keystrokes are sent back to the KeySweeper operator over the Internet via an optional GSM chip, or can be stored on a flash chip and delivered wirelessly when a secondary KeySweeper device comes within wireless range of the target KeySweeper. A web based tool allows live keystroke monitoring.

via KeySweeper.