I'll try to list only stuff that others haven't mentioned already (if someone posts one of these while typing this out, fuck it, I don't care :P ):

Before The Echo (formerly known as Sequence before they changed the name), being a rhythm game has a really nice soundtrack with quite a bit of variety in it (I mean, I have no idea if the variety is too big- they just sound mostly different to me since I don't know shit about music genres XD ).

Castle Crashers' soundtrack was done by a bunch of different... um... composers? Is that the right word? Well, anyway, it has a lot of variety to it too and it's pretty good. :D

Most of the Hyperdimension Neptunia soundtrack is really nice (and the games are great, try them out), but I guess out of context I'd suggest this one (cuz I can't stop listening to it).

Serious Sam, being an awesome series, has some super awesome action-y soundtracks too(last one also has a version with vocals if you prefer that stuff).

If Serious Sam isn't metal enough, there is always Painkiller. Fuck yeah. :D

Since I'm on a FPS wave- can't forget Hard Reset. Has a futuristic sound to it because robots (unfortunately I was unable to find a high quality version of the final final boss' theme since they seem to have never released the expansion's soundtrack :'( ).

Btw, why hasn't anyone mentioned Half-Life 2 (or Half-Life in general) yet. I know most of the soundtrack is ambience but the stuff that aren't are awesome (and the ambient stuff is perfectly used in-game)! :D

Devil May Cry series. Yes, all of them! XD

Giana Sisters: Twisted Dreams. I don't even know where to start suggesting individual parts of this one. All of it is great (heck, even the menu and credits music; also all the themes except those two have 2 versions of them, both of which are always awesome). Also the standalone expansion makes some equally nice additions to the soundtrack. :D

Broforce has some nice ones.

Ok, those were the more "mainstream" ones. Now for games that pretty much no-one else would ever mention since they are super niche. Yes, I am talking about Bullet Hell games. They usually have awesome soundtracks but very few people play then. :/
Anyhow, in no particular order:

QP Shooting - Dangerous!!- small but gold.

The entire Gundemonium Collection. That's 3 games: Gundemonium Recollection (yes, that is a stupid remake name), GundeadliGne and Hitogata Happa. They're all really good games (though Hitogata Happa will murder you horribly). :D

Jamestown. A nice little game.

Rabi-Ribi. A Metroidvania/Brawler/Platformer/Bullet Hell hybrid. I guess they weren't being picky when they were squeezing down the potential fan-base since the game is so awesome. And its soundtrack is just as awesome. :D

This is more of a honorable mention since I've haven't played Sora (yet), but thanks, Youtube, for recommending me something nice for a change. ^_^'

Fuf... I think I've exhausted most of the video game soundtracks I can recommend that weren't recommended yet. Anyhow, hope you like at least some of them. :P

VVVVVV
Crypt of the Necrodancer
Dust: an elysian tail

I loved the soundtrack for The Charnel House Trilogy. It fit the game very well, and also stands as rather good ambient music on its own.

Surprised not to see

Silent Hill 2 Even the music puts you in this mood of dread and loneliness.
Fallout: New Vegas nuff said.
Machinarium great soundtrack for a great little game, when is cloudy and rainy i always put this one on.

Trace Vector

I agree with a lot of the previous games mentioned, but would add They Bleed Pixels to the list. I was humming that soundtrack for weeks.

RA2 and FF8

I weak against old games xD
Old games are really so lovely, I can see people behind it really enjoying themself while making the games

The only full video game soundtrack I can recall listening to is Journey. It fits the aesthetic of the game perfectly and was the first video game soundtrack nominated for a Grammy. :)
Most of the soundtracks I listen to are movie soundtracks but there's some game soundtracks too. Oh! The Last of Us soundtrack is really good too (even though I've only seen a play through of the game and haven't actually played it myself)

The Beginner's Guide

I love the music for Remember Me, but it unfortunately does not have a soundtrack for it. Which is a shame, as I'd buy it in a heartbeat.

The soundtrack to Arcanum is one of my all time favourites - it matches the game perfectly and I very nice on it's own, if you're into that kind of stuff (it's almost entirely a string quartet).

I also like Bards tale & Bastion.

I love the soundtrack of Electronic Super Joy and its sequel/spin-off Grove City, I actually have both soundtracks on my phone and listen to them sometimes.

Game soundtracks I listen outside the game? Plenty.

Advent Rising, practically all Final Fantasy games since FFIV (mostly FFIX, FFXIII, FFX-2, and FFVI), Remember Me, Castlevania franchise form CV IV and on (primarily Harmony of Despair), Prince of Persia: The Sands of Time, Star Wars: Republic Commando, Super Mario Galaxy 1-2, Chrono Trigger, Legends of Mana, Bastion, Transistor… and I am sure I am missing some, but my music library is back at home so I cannot check.

Some good jazzy soundtrack :
Contrast
LA Noire
(Excellent games by the way)

Hyper Light Drifter
Icewind Dale 2
Baldur's Gate 2
Hired Guns

Baldur's Gate YouTube link has one of my favourite soundtracks - I don't know whether the enhanced edition stuff is any good - but the original OST is still a album I go back to regularly.

I don't really care about game soundtracks in general, but I do love some soundtracks of movies that are absolutely shit. Just the soundtracks, nothing else.

Ohh yeah, I forgot about the game I am currently playing Planescape: Torment, it has some very nice themes. This one is really nice (it`s looped in this video so you can stop listening after about a minute and a half if you get bored though I really like it)

https://www.youtube.com/watch?v=HCpk7HIryK0

BTW. HELL YES for the Bard˛s Tale hahaha. What a funny game. :D
Fallout 3 is awesome as well-
DOOM - I cant remember any game that I`ve played that had a metal soundtrack (Other than Wolfenstein: The New Order - but that was also composed by Mick Gordon. ). When BFG kicks in it feels so damn good running around killing demons haha

Most of games have great soundtracks, worthy to be listened separately. I'll post the latest one, which I loved both during gaming, as it fitted perfectly, and during spare time:

Mother Russia Bleeds

Brutal Legend, NFSU, NFSU2, NFSMW (2005)

I don't listen to music on its own, but when I game mastered Dark Heresy I had the Chaos gate soundtrack on (on low volume) in the background.

Witcher 3 (Base+HoS+BaW)
Terraria
Hotline Miami 1+2
Transistor
Bastion
Jet Set Radio
Stardew Valley
Fallout 3, 4, New Vegas

and many others I can't remember off the top of my head and or still need to listen to.

It's a secret!
And the beat goes on