http://www.captainforever.com/captainforever.php
Essentially, you pilot a little ship and find different modules you can stick together to build an even bigger ship with better guns. More ships come in with better modules, so you kill em and take em. You have to balance out thrusters and weapons and not build your ship all lopsided and underpowered, or else some CPU jagoff will make you look like a chump.
So fun.