In the Gran Turismo 5 Prologue game for PS3, there are several bonus cars you can unlock by completing certain tasks.
Daihatsu OFC-1 – Win all of the races in the C Class
BMW Concept 1 Series tii ’07 – Win all of the races in the B Class
Nissan Skyline Coupe Concept ’07 – Win all of the races in the A Class
Nissan GT-R Proto ’05 – Win all of the races in the S Class
Ferrari F2007 – Win all of the races in the C, B, A, and S Classes