There are a million places online you can download every midi or CD soundtrack for every single Pokemon game so I'm going to focus on my favorite Pokemon songs here. I hope you download a few and that listening to them makes your day a little better.

Otsukimi-yama (Mt. Moon)

Tokiwa Forest   (Viridian Forest)

Show Me Around

Tamamushi Game Corner

Rocket-Dan Hideout

Rocket-Dan (GSC)

The End

Silent Hills (Spaceworld Beta, MP3)

Village Bridge (MP3)

Kagome Town (Lacunosa Town, MP3)