Of course, while licencing the right song for a game can add to it, making the right soundtrack for it is even better. There's millions of video game tracks out there, and even more good ones than one post can handle. So, for this edition, I'll just focus on a few of my favorites.
THE FIGHT THEME FROM PUNCH-OUT!! (NES)
So, you think you can beat Mike Tyson? If not, you will after this starts playing. It'll pump you up perfectly for the road ahead of you, changing slightly every fight, getting more and more intense as you climb the ranks. It'll get you right into the zone you need to be, and when you hit a roadblock (or King Hippo), it'll do it's best to keep you from giving up, making sure your burning heart stays on fire and you keep your tiger's eye focused on finding the way to win.
And if you still doubt that it's a fantastic track, listen to this version and feel the doubt melt away: http://ocremix.org/remix/OCR00165/
THE THEME FROM BUBBLE BOBBLE (ARC)
If you were in the middle of a fantastic journey through the cave of the monsters, and could only take one song with you, why not make it cheery and upbeat? Sure, it'll be playing through every single one of the one hundred levels of the cave, but it's got this charm to it that makes it hard to get tired of it. It's great in game, it'll get stuck in your head for a looooong time after playing, and it's even pleasurable to listen to outside the game.
SUPER MARIO LAND- EASTON KINGDOM (GB)
Everyone probably could recite any tune from one of the main series games by heart without even thinking. Maybe a few less could put out one from one of the Game Boy editions of the franchise. While the first level theme has achieved a level of popularity, this second level theme is, similar to the Marble Zone from Sonic 1, vastly underestimated. I'd argue it's even a more enjoyable tune than some of the NES ones. It fits it's level well, it's really catchy, and is liable to really get stuck in your head for a bit after playing.
SIM CITY- CAPITOL
This one could just be nostalgia talking, but I love this track. It's this soft, yet kind of bustling track, like morning has dawned on a city. If I could sum it up in a way everyone would understand, it wounds like something you'd hear during Mr. Rogers Neighborhood. It really fits the pace and tone of the game, especially during this phase of the city. Nothing too intense, but still enough to keep you in the mood.
Of course, these are only a few that I'm remembering, I'm sure there's tons more that I'm not even thinking of. Feel free to post some of your favorite original game tunes in the comments!