Happy birthday Ross! Ok, you probably have a lot of recommendations to go through so i'll try to keep this as brief as possible. These games aren't exactly lesser known (to be honest they're pretty mainstream) but whatever, good music is good music.
Halo 3 ODST
(To download file, click VBR mp3 files on the right, right click song at choose save file as)
Rain (deference for darkness)
Still grounded (asphalt and absolution)
Where one road ends (neon night)
Street sweeper (traffic jam at 0:48)
From the ashes (bits and pieces)
Air traffic control (skyline at 4:36)
Cast aside (no stone unturned)
Orbital drop shock trooper (finale at 1:12)
This game may be short (3-6 hours) but the soundtrack is what sold it for me. The composer, Martin O'Donnell, has done an amazing job with the series but this soundtrack in particular stands out as it uses none of the themes from the other games, giving him a lot more freedom. With soft strings, piano and saxophone tunes contrasted by the drums and electric guitar that play during the big battles, this soundtrack keeps me coming back.
Ace Combat X
Prelude, joint operation
Pinned down, in pursuit II
Time limit (this one is kinda spooky)
Gauss tower, wild card
End of deception
Alect squadron, armada, operation x
Strategic briefing (one of the lost tracks, may not be in list of songs)
Someone has already mentioned the Ace Combat series (would definitely recommend 4,5 and zero), but they missed out this gem. Ace Combat X is a PSP exclusive, so it's likely you probably haven't played it. If you have listened to the other Ace Combat OSTs, this one is a bit more electronic like Ace Combat 3, but still has lots of orchestral parts. Luckily, someone has ripped the sound files from the game, but some of the songs may not be in the link. I've only found them on youtube (called the lost tracks).
Theme of SSS
This is from an anime, not a game but someone has already suggested anime and I thought what the heck. Listen to this or ignore it, I won't be mad. I promise. Anyway, the soundtrack takes inspiration from RPG music. There is a lot of piano, A LOT, so if you like that you're in luck. I've recommended the few songs i'm sure you'll like, plus decisive battle sounds like a pianist is having a seizure.
Ok, there a lot of pokemon games (and not just the main games), which means there are a lot of songs I could recommend. However, it would be better if I did that in another post when I have time, so stay tuned.