Firstly, each game has it's own type of music I think... I am saying, that you can't put a rock music in a love game(or scene). How does that look like? The lovers are kissing, and a loud black metal music is playing in the background? I think that would be horrible, or hillarious! Let's see, which game, need which music!

ACTION GAMES
It depends on the scene:
If it is a shooting scene, then you can use ROCK music. ROCK music goes very well with the gunshots, and with the hard action...
If it is a love scene(you know what I mean, the thing that girls and boys do, in a bed...), then you can use ROMANTIC, or SLOW music(like Bryan Adams - All for Love), or POP music, but the slower ones.
If it is a sword fighting scene, then the best music is the ROCK music I think(I think rock music goes with many games)
If it is a secret agent style scene, then you can use musics like Mission Impossible Theme(if you don't know what I mean, buy the Limb Bizkit album, with the music: Take a Look Around, that's the theme for MI)

STRATEGY GAMES
For a strategi game, use musics that pump up the adrenaline, but not very much... when there is a fighting scene you can use ROCK music, when everything is silent, you can use MELANCOLIC music, like Apocalyptica.

PLATFORMERS
It depends on the type:
For a silly platformer game, the silly type of music goes(like Mario, I shouldn't say silly, I should say, VERY IREALISTIC), for a shooting platform game, like Contra, I suggest ROCK music(it pumps up the adrenaline).

HORROR
Definitly MELANCOLIC, CREEPY or ROCK music. For the action scenes ROCK, for every other scene MELANCOLIC, but especialy CREEPY.

FARM SHOOTING GAMES(like Moorhunt, and like Bad Billy, you can find Bad Billy on TDC, in the downloads)
Definitly COUNTRY music.

If you don't know where to download ROCK, COUNTRY, POP, RAP, or any other type of music, then here is a site what I use: www.midifarm.com < from here you can download musics from different artists... have fun choosing your music to your game, and choose well, because I think music is one of the most important things in a game... it gives a good feeling to the games... this is all. Have a nice day!