I've found the Stronghold soundtrack to work perfectly while playing Minecraft. The Betrayal At Krondor soundtrack is also great but it hasn't been added as an extra. It's free though so you should download the songs manually from Quest Studios. Jan Paul Moorhead is the composer.

The soundtrack for the my favorite medieval game, Knights of Honor is also worth mentioning. Unfortunately the game isn't available on GoG (it's on Steam). The main site (who hosted the free soundtrack) is down but fans of the game are hosting the soundtrack via rapidshare themselves. Links can be obtained here: http://forums.blackseastudios.com/archive/index.php/t-190.html

If/When GOG adds the sndtrack from Age Of Wonders, i guess it'll be worthy to be added to the "best", even with that Reyn Ouwehand tune played backwards.

I completely forgot about Outcast. Outcast sounds like a truly epic movie that was never made.

Earthworm Jim 1 and 2.

Tropico Reloaded!!

Darkstar One's is pretty good and so is Baldur's Gate's.

Total Annihilation
Interstate 76 (if that counts, seeing as the soundtrack isn't included).
Tropico Reloaded
Outcast
Divine Divinity

The Descent 2 redbook soundtrack (and the Vertigo expansion, which isn't on GOG).
I haven't bought the GOG version, but I remember reading that the GOG version was missing the CD music, hopefully they have fixed that by now - otherwise you're really missing out.
It's got awesome music by Mark Walk/Ogre of Skinny Puppy - a.k.a OhGr/Download, and Type O Negative.

The Total Annihilation soundtrack is fantastic as well.

Download my compilations of various GOG game soundtracks. 192kbps VBR MP3.

Note: These are not GOG bonus materials! I compiled these on my own accord.

Interstate '76 (Converted directly from a FLAC rip, appropriately tagged)

Rise of the Triad (Converted from Brandon Blume's SC-55 rip, appropriately tagged)

More soundtracks at my VGM Twitter account.

Agree with Tropico Reloaded and Rise of the Triad, just forgot to mention them earlier. But they are indeed great!

Tropico Reloaded's soundtrack is fantastic. And Republic the Revolution doesn't just have a soundtrack of excellent kind of martial sounding, anthemic Eastern European music, it's better than the game. :)

Here's a quick summary of the advised game soundtracks, in order of recommendations. Note: I've only icluded games which have downloadable soundtracks as part of the GOG freebie package.

Total Annihilation: 7 votes
Tropico Reloaded: 4 votes
Heroes of Might and Magic 3; Outcast: 3 votes
Baldur's Gate; Evil Genius; Fallout; Heroes of Might and Magic 2; Rise of the Triad: 2 votes
Beyond Good&Evil; Blood; Darkstar One; Divine Divinity; Earthworm Jim 1+2; Republic: The Revolution; Stronghold; Stronghold Crusader; The Longes Journey: 1 vote

The Descent 2 redbook soundtrack IS fantastic. Unfortunately you're correct, it isn't included with the GOG version, or rather it is, but not as a separate download. Fortunately, the actual redbook audio is included as a CD image (which is used to play the music in-game). It's very easy to rip the music yourself. The album database that WMP uses even has the track information.

To extract the music, simply make a copy of the file DESCENT_II.inst in the Descent 2 folder and rename it to DESCENT_II.cue. Then mount it in Daemon Tools or whatever virtual drive software you prefer. After that it's just a matter of ripping it like you would any CD.

It took me all of 5 minutes to do the above, including finding the cover art and slapping it on the ripped album. I reckon GOG could easily do the same and put it up for download. The same goes for any game that uses redbook audio.

Common Total Annihilation's OST is just perfect for the game, Jeremy Soule is an amazing composer , the best thing in Icewind Dale was his music...
Seems like he also wrote Duke Nukem Forever's soundtrack , just one more reason to get existed about it !

O' and tho you can't download it's soundtrack from GOG , Advent Rising OST is beyond incredible!
"Bounty Hunter" is one of the best tracks I've ever heard period.-

http://www.youtube.com/watch?v=EQ_hgmGEhsw&feature=related