Gustav Holst – The Planets
Anyone can tell you that Mars: The Bringer of War is awesome for an epic battle. The less-used known Uranus: The Magician is the optimal boss-fight music, and makes your bosses feel twice as evil and four times as dangerous. Use Saturn: The Bringer of Old Age for a battle against an ancient and mysterious creature, and Neptune: The Mystic for mysterious ambiance. Jupiter: The Bringer of Jollity is a good “victory” tune for returning heroes.

Modest Mussorgsky – Pictures at an Exhibition
A really great suite with lots and lots of stuff you can use. Gnomus is great for fighting ghosts or vampires. Bydlo is great for an epic dungeon, Le Cabanne sur des pattes de poulle is good for an intense battle, and Le Grande Porte de Kiev is another great “victory” fanfare. If you’re not afraid of using something your players will recognize, Mussorgsky also composed A Night on Bald Mountain, which is almost TOO epic.

Philip Glass – Glassworks
This is really weird stuff. You might have heard some of Philip Glass’s similar work in Watchmen when they were going through Dr. Manhattan’s transformation. I like it for weird ambiance, and sometimes for a fight against a strange or alien creature.

Antonin Dvorak – Slavonic Dances
These are great for dramatic scenes. They all feel a little bit similar, but they have a pretty huge range of intensity, and are great for a dramatic swell during a major plot event.

Carl Oorf: Carmina Burana
Obviously O Fortuna works, but there are a lot of lesser-known Carmina Burana selections that make for great battle music. Fortune Plago Vulnera is good for almost any combat situation. Ecce Gratum is good for a celebration, Tanz is fun for a light combat or adventuresome non-combat situation, Floret Silva is good for a lighthearted, but high-stakes combat, Were Diu Werlt Alle Min is good for victories, celebrations, and introducing epic scenery, In Taberna Quando Sumus is good for any intense situation, and hey, I’m gonna quit there, because I’ve named most of the songs in the suite.

Anyway, there’s lots of other stuff that I haven’t discovered yet. Try your local library! They usually take pride in having tons of classical music, because it seems like the proper music for a library to have.

A great tool (originally designed for podcasters) is CastBlaster. This tool allows you to setup audio tracks so they can be clicked on, or you can use keyboard shortcuts. You can also record things with it, using all of your sound effects to record a meta-track. I’m currently in the process of running the finale sessions of a GURPS space campaign, and built a track that had a good soft epic feel, to play behind a politician giving a speech in a park to thousands of people. I planted huge explosions in the audio track at a pre-defined time. I had practiced up the section a bit to make sure that i wouldn’t run out of speech content, and in the middle, BOOM, the govt. building behind the politician explodes and knocks everyone on the stage down.. I added ambient crowd screaming to the track after the explosion.

The look on my players faces as a huge explosion sound followed by a crowd screaming was priceless!

There’s a ton of free sound effects sites online, to get ambient tracks, like nature sounds as well.

From a background perspective, I am currently really liking Clint Mansell’s ‘The Fountain’ soundtrack. It’s really cool, and doesn’t have a ton of the huge changes in a track, where volume swells and dips dramatically.

My $0.02

First Knight
Robin Hood: Prince of Thieves
The 13th Warrior (also good for spooky atmospheric music)
Willow
Chronicles of Narnia: The Lion, The Witch, and the Wardrobe
Chronicles of Narnia: Prince Caspian
DragonHeart

I followed the OP’s convention of breaking everything down into four categories: action, ambient, triumphant, and forboding. These are my findings after going through many online, movie, and game sources.

Online: Celestial Aeon Project (hands down the best – great for all categories listed), and David John’s free tracks (posted above, very nice for Fantasy settings).

Movies: Bram Stoker’s Dracula (quiet at parts, but when it kicks in it can get really creepy – great for foreboding), Gladiator (mainly ambient, but very heroic/epic sounding), Resident Evil (the soundtrack is mostly songs with lyrics, but the last 4 tracks are score pieces created by Marilyn Manson – they are great for action/foreboding if you can fit industrial sounding music into your campaign), The Fountain (awesome for ambient, some really great tracks here), Beowulf (if you want pompous heroic themes, search no further), 28 Days Later (awesome foreboding and ambient tracks).

Games: Heroes of Might and Magic (mix of ambient, action, and triumphant), Elder Scrolls (same as HMM), the Myst series (awesome for ambient, with a bit of foreboding as well), Silent Hill (very creepy, and great for foreboding, with some ambient as well), Final Fantasy VII: Advent Children (if you can handle some distorted guitar, this is *great* stuff for action, and also has a bit of the other categories), Diablo series (awesome ambient/action), Resident Evil series (foreboding, action, ambient).

Music: Blackmore’s Night (as Swordgleam suggested, great for Fantasy settings – go for the songs without lyrics for your campaign, though).

Hope this might help someone narrow down their search! My biggest difficulty was finding music that fit into the triumphant category. Any suggestions there?

And, of course, you can use iTunes to set up looping and repeats and random shuffles and so on.

I think ‘in town’ vs ‘in adventure’ musical cues are great and do set a mood. I tend for more classical stuff, and I will suggest anything by Michael Kamen (Band of Brothers is very good for longer, harrowing combats, as is the original Highlander).

I was flipping through channels a couple of weeks ago and Conan was on. I wasn’t even paying attention, but the music instantly grabbed me — it was totally Pavlovian. My dice hand started to itch, and I suddenly craved fun-yuns and Shasta soda. I had to go dig out my dice and peer over them