Tyrian is a vertical-scrolling shoot-em-up published by Epic MegaGames, in 1994. The full version retailed for $30, but Epic released an excellent shareware version. Tyrian was actually one of the first games to have a semi-decent story as well as bits of humour thrown in.
The story is that, in the year 2xxxx, an anti-gravity mineral has been discovered on the planet Tyrian.
Microsoft MicroSol has stepped in to control mining of this rare mineral, and Trent just happens to be caught in the middle of it; you escape from MicroSol's prison camps and thus begins your quest to bring down this evil corporation. Much of the story comes in the form of 'datacubes' collected by destroying enemy ships.
Your score was used as currency, operating on the idea that a higher score would be netted by destroying more ships. Between each level, you were given the opportunity to upgrade your ship, weapons, and add special weapons called 'sidekicks'. Sidekicks flew on either side of your ships and assisted in adding to your total firepower.
The ship was extremely customisable; the lifeblood of your ship, however, is the generator. If you installed a poor generator, it would adversely affect your ability to fire and to replenish your shields - once your shields were whittled down, your defense consisted of your ship's armor. Both your ship's shield and generator continually recharged, but you are often under heavy attack most of the game.
A unique feature of this game is the ability to do 'twiddles', or 'Street-fighter-like moves'. Depending on the type of ship you were currently piloting, you could replenish your armor, send a wide swath of destruction forward, or do another cool trick. This was added in later revisions.
As you progress through the game's levels, eventually you'll find out that by pressing spacebar, your ship would literally wipe everything out onscreen because of the sheer firepower that you've accumulated. And even that won't protect you from the bosses in the registered version
Recently, Tyrian 2000 was released - but it just doesn't have that same charm that the original game did.