On nothing to do with Dopplr ::
http://arduino.cc (edge connectors), wii controllers,
http://johnnylee.net, $5 remote control car, remove cable control with bluetooth, tenori-on (synthesiser designed for playing with music, midi out, performance, leds covered, Toshio Iwai stage performance with visual at rear), arduino connected to mobile with python running app, oystercards with rfid reader £30 read from unique id of oyster used for voting, bbc dabagotchi (pda in toy with usb, squeeze hand if like its music, dies if don't use it), laser harp (
http://stephenhobley.com/build), nintendo ds (mic, touchscreen, interface, edge connector), ds Brut use nintendo homebrew kit to create and physical outputs, think about nodes and enhancing digital - "Making Things Talk" electronics book Matt recommends,
http://tinker.it consultancy which will create physical objects from your games.