I've been recently listening a lot to music from different games. Aquaria, Penumbra, Cave Story, Metroid Prime (2), Abe's Oddysee/Exoddus...
There's many relaxing songs in games, plus those epic boss musics. holy shit.
Pretty much why I like it. :v

Anyways, if you know any good tracks from games, post them here!
Here's a "few" of what I think are quite good...

Aquaria OST - Open Waters 1
Aquaria OST - Remains
Aquaria OST - Worship

Penumbra: Overture Theme
Penumbra: Requiem Ending Theme
Penumbra: Black Plague OST - Danger Is Near

Metroid Prime OST - Underwater Frigate
Metroid Prime OST - Space Pirates Battle
Metroid Prime OST - Thardus Battle

Metroid Prime 2: Echoes OST - Ing Battle
Metroid Prime 2: Echoes OST - Dark Samus Battle
Metroid Prime 2: Echoes OST - Vs. Emperor Ing 2

Cave Story OST - Mischievous Robot
Cave Story OST - Moonsong
Cave Story OST - Oppression

I'd really like to post a lot more, but... Let's keep it reasonable. At least somehow.