Keysweeper is a super-creepy keystroke logger disguised as a USB wall charger that piggybacks on GSM networks.
Its developer, Samy Kamkar, describes it as a "stealthy Arduino-based device, camouflaged as a functioning USB wall charger, that wirelessly and passively sniffs, decrypts, logs and reports back (over GSM) all keystrokes from any Microsoft wireless keyboard in the vicinity."
It logs keystrokes online and locally, and the user can set up SMS alerts to be sent when certain trigger words, usernames or URLs are sent, to better identify passwords.
"If unplugged, KeySweeper continues to operate using its internal battery and auto-recharges upon repowering. A web based tool allows live keystroke monitoring."
More: Keysweeper [Samy Kamkar via Robert Caruso]