A homebrewed hybrid of Gravitar, Oids and Thrust, Gravitron uses a similar glowing-vector style to Thrust Xtreme for its visuals, but is more varied in its environments and elements. In addition to tight caverns to navigate there are spacemen to rescue, reactors to destroy, boulders to clear out, enemy ships, forcefields and a variety of gun emplacements to tackle. It can get pretty intense, and luckily there's a password system that allows you to start over from any level you've previously reached.
Suffice to say I'm won over by the nature of the game itself and the visuals. This is like a game created with the sole intention of appealing to me, and it's a no-brainer for inclusion in my 100 favourites.