All of the following are from metroidvania style indie games you've probably never heard of.
Hero Core Soundtrack: http://brotherandroid.com/album/hero-core-ost-stereo-expanded-edition
Very fun freeware game. It's a 2D shooter where you float around freely killing baddies, bosses, and collecting power ups. Notable for many reasons: It's the only game I've played where each difficulty has it's complete own map, it also has different enemy and boss patterns depending on the difficulty.
Hydra Castle Labyrinth Soundtrack: https://www.dropbox.com/s/18e51dhgxrtdtfe/Hydra%20Castle%20Labyrinth%20Soundtrack.zip?dl=0
A small freeware platformer based on Maze of Galious on the MSX computer in japan. It's a very fun exploration game, with a stellar soundtrack, which is bafflingly made entirely in midi. Since the soundtrack isn't available anywhere, I've left a dropbox with the original midis. For best effect use the default windows midi sequencer.
Treasure Adventure Game Soundtrack: https://robitstudios.bandcamp.com/album/treasure-adventure-game-ost
A surprisingly large freeware platformer evoking memories of The Legend of Zelda the Wind Waker with a (not freeware) remake coming out soon. My three favorite tracks are Fight or Fight, Duct Work (Electrified), and All Things... Notable for having the highest concentration of plot twists in 10 minutes I've ever seen in a game.
Finally the La-Mulana Soundtrack: https://nigoro.bandcamp.com/album/la-mulana-original-sound-track-complete and also https://www.dropbox.com/s/f3i3996shufawod/La-Mulana%20Soundtrack.zip?dl=0
La-Mulana is my vote for best indie game of all time, possibly best game of all time. A massive platformer with a heavy emphasis on cryptic and difficult puzzles. The puzzles mostly involve solving riddles and paying close attention to the environments in the game. They're hard, but also fair, however there's definitely no shame in using a walkthrough. There's the original freeware version and also a not free steam remake. The remake generally has slightly easier puzzles and slightly harder gameplay. Both are fantastic but overall I recommend the remake, though there are enough differences to make playing both worth it. It also has an awesome backstory involving aliens and most religions, past and present with a surprising amount of research put into it. The bandcamp is for the remake soundtrack, and the dropbox has a few songs I like more in the original soundtrack.