In Burnout Paradise, depending on what you do in the main world of the game, you can unlock different cars. Some of these are listed below.

Carson Carbon GT Concept: Find all 400 smash gates
Carson Fastback: Get a B license
Hunter Mesquite: Get a D license
Jansen Carbon X12: Land all 50 Superjumps
Montgomery Carbon Hawker: Hit all 120 Billboards
Nakamura SI-7: Get a C license
Rossolini Tempesta: Get an A license