Someone who plays first person shooters like Quake, using a mouse or trackball to turn, whilst moving with the keyboard (or, sometimes, a joystick) Mousers are often looked down in contempt by lame keyboarders who stupidly think that they use the mouse to move, until they actually play a game against one that is. By definition, a mouser is better than a keyboarder. All Quake demigods are mousers.
See also : Thresh