Convoy Assault

Convoy Assault is a game in which you navigate your spaceship to fight enemy ships, collect pickups, and destroy increasingly dangerous convoys.  The game gets more complex over time, with enemy hives (space stations), pickup-eating snakes, space warps, and more.  You can unlock additional pickups by achieving various milestones.  It's sort of like being a U-boat trying to disrupt Atlantic shipping, only more zany and in space.

Single player and local co-op (2 player split-screen) modes.  Harder difficulty levels effectively start you farther into the game.