It's funny. Geometry Wars was originally developed as a small project to test out the 360's controller, and now, it turned out to be one of the most addicting games available.
The core concept is simple. Players control space ships and are tasked with avoiding violent polygons hurling towards them. They use the left analog stick to move, and the right to shoot. The goal is to survive for as long as you possibly can.
However, it's not the concept that makes this game stand apart from others, it's how polished it is. Geometry Wars was a decent title, but Bizarre Creations improved all the right areas in Geometry Wars: Retro Evolved 2.
Take that you filthy abhorrent shapes!
Currently, Geometry Wars: Retro Evolved 2 holds the title as "The Best Xbox Live Arcade Game," and while I'm not going to argue with that, I think a better — and more fitting title — would be, the "Best Stress Reliever."