NOIZ2 (web/Java - free)

When I was little and bored in synagogue or school, I'd squint my eyes so the world became flat and carefully track the path of an imaginary infinitesimal dot as it bounced around off whatever edges were still visible.

Now imagine a cross between this admittedly senseless pastime and something like Raiden, written in Java. From a site recommended by Penny Arcade: Noiz2. 'You' are the tip of your mouse pointer, dodging missiles and trying to pick up green stars. When the flickery line leading upward from your pointer crosses an enemy, a split second later you automatically zap the enemy with a laser beam.

Simple! But the momentary gap between lining up with a bad guy and actually firing makes it satisfyingly thuddy; I can just hear the (absent) WHUMP of space ordnance colliding with its target. Combined with the odd airiness of having no body in the game (and being able to move as fluidly as the mouse lets you) it's hypnotic. Highly recommended.