There are several tracks you can unlock in Gran Turismo 5 for the PS3 for use in Practice and Arcade modes.
Here is a list of unlockable tracks, and what you have to do to unlock them:
Nürburgring Nordschleife – Get bronze on all of the Intermediate AMG Challenges
Nürburgring Nordschleife 4 hour circuit – Get silver on all of the Intermediate AMG Challenges
Nürburgring Nordschleife 24 hour circuit – Get gold on all of the Intermediate AMG Challenges
Top Gear Test Track – Get gold on the the Beginner Top Gear Challange
