Monect - Make the best use of sensors on smart device to control PC, simulate steering wheel, game-pad...
LEA Extended Input - iOS/Android app with windows server to control functions in games via your smartphone/tablet.
Wifipad - WifiPad transforms your iPhone/iPod Touch into a wireless gamepad.
Joypad - Turn your iPhone into a wireless game controller!
nJoy - Complete PC remote controlling solution for Android.
DroidPad - DroidPad lets you use your Android powered phone as a Joystick for your PC.