I absolutely adore the game design of Tempest, and while the original still looks nice and plays a decent game, the idea really shone when Jeff Minter took it and gave it a total makeover for the Jaguar. Since then it's gone through many incarnations on many platforms, but it's retained that manic eye-melting formula. Typhoon 2001 - a freeware, homebrewed tribute - is the finest version I've played yet.
It's the kind of game that only makes sense to the person playing it (and even then only some of the time). It's completely exhilarating, and sometimes overwhelmingly frantic. Typhoon 2001 throws down the highscore gauntlet and dares you to blink. It's arcade gaming mainlined into your brain, forcing your hands to play catch-up. It's my absolute favourite dose-of-adrenaline game. The surest way into The Zone.