I just picked Tyrian 2000 up free from GoodOldGames.com, a site that legally sells DOS games packaged with DOSBox in a manner to make them easy to use in newer Windows operating systems. I've dabbled in OpenTyrian, and even Tyrian, and have to say, if Tyrian is in the name of it, you probably want to play it.
Tyrian is a vertical scrolling shooter, which, like most of the genre, pits you in a never-ending battle against fast enemies, but allows you to spam fire uncontrolled until you kill something. I've had an odd fascination with this sort of game, and felt compelled to try it out. For a fifteen-year-old game, this one still has some life left in it to give, especially since it's also got a free, open-source clone named OpenTyrian, that runs a lot lighter than Tyrian in DOSBox. I personally think Tyrian 2000 in DOSBox is the best game as it's Tyrian with one extra scenario, and slightly better sounding music, and an extra mode or two, but if your computer lacks the punch required for the best experience possible, you can still play OpenTyrian.