Old school shooters like R-Type or Xevious have a degenerate form of gameplay: the bad guys shoot swarms of very slow moving bullets at you and the challenge is to find a path through the swarm that leaves you safe. It's hypnotic and simple.
Tuning the bullet swarm is a fine art. So why not describe bullet swarms with XML? BulletML does exactly that. Try the Java applet (select a file other than template.xml, press start, move mouse in upper left window). Or play a stylish Windows shooter that uses BulletML: rRootage. (There's a Mac version, too.) rRootage goes further with Bulletsmorph, using genetic programming to come up with new bullet swarms. Crazy.
As seen on Molelog