Today's great PC game you've probably never heard of? The Polynomial. Or, as I like to call it, my "just got home from the pub" game.
It's a mathematically-generated space shooter. How's that work? It plays music in the background, of your choosing, then adjusts the surrounding space to match the rhythm of the track. Like a music player's visualiser mode, basically, only with stuff to shoot in between.
If, like me, you could actually do without the shooting and just want to fly around, you can remove all the bad guys and just cruise, editing the rules governing the generated display to create your own space art.
The game's official site is below, and you can grab it for $10 off Steam. I've put together a short demo reel showing you how it reacts to different types of music on the one setting (you can of course tinker with things to achieve radically different results).