All Pokémon Emerald music.

The opening theme for Secret of Mana always get me nostalgic.

Teenage Mutant Ninja Koopas Attack on the Koopa Bros from Paper Mario

Yoshi's Island - every track in this game fits so well with its level that I could guess the track used from a single screenshot. The outdoor levels are upbeat and calming, to give you that feel of being on an actual island. The jungle portions are more zany to match the different enemy types. And the boss level themes did a great job of making it feel like you were sneaking through a fortress. I will never not love this soundtrack.

Too many to list, but here's a few notable examples:

The Legend of Dragoon:

Final Fantasy IX:

Freespace 2:

Total Annihilation:

There are way too many to list, but here are some of my favorite nostalgia-filled tunes:

Pokémon Gold:

Super Paper Mario:

Mario Kart Wii:

And finally, my most nostalgic piece of videogame music, this song is the definition of childhood to me:

anything from xenogears, Donkey kong country trilogy and megaman x

Oh man this takes me back, add on this and you got my childhood.

FF7 Prelude

Can people please stop posting a 10 youtube videos. It's crashing browsers and makes this topic very unresponsive. Here is a  tip. Posting 1-2 encourages others to post one giving you a chance to post another. Helps the topic live longer.