When you go into the showroom there are 5 cars under tarps in the bonus area that you cant even test drive. Each car is unlocked by how well you complete the game. Here is a list of what unlocks what:

All Silver Medals or Better: Porsche 911 GT1
All Steel Medals or Better: Delfino Feroce
All Bronze Medals or Better: Ferrari 250 GTO
All Gold Medals or Better: Mercedes CLK-GTR
All Platinum Medals: TVR Cerbera Speed 12