Symphony! I posted this previously on another shmup thread, I got this off of GOG but you can also get it on Steam
click here -
Yeah, so onto Symphony. I had no idea this was an indie game, but boy is it great.
Each "level" is generated by music files on your computer (and not just MP3s either, I found out, it accepts a few extra formats) and the enemies are dynamically generated by the music, and the more intense the music is, the harder the enemies will be. After every stage, you unlock a new weapon to purchase, and you have 4 "guns" on your ship to assign them to (or you can upgrade a current weapon). One in a while one of the 5 "demons" in the game will challenge you to a boss battle as you're playing, and defeating them gets you a piece to a song that will "liberate your music" from its evil control.
It's no bullet hell, it's more of like an old school style presentation with really nice looking vector style graphics, but there are features such as chain scoring, online leaderboards and most of the modern amenities that newer games have.
$10 on your digital download service of choice. Highly recommended!