I found this site recenty, The internet movie car database. Basically it tells you what cars, vans, trucks bikes etc were in which films & TV programmes. I can't vouch for it's accuracy but I can see it beinga good way to end arguments (sorry, heated discussions). Ever wondered what was that car in the background in La Boulangerie de Moncea? It's a Simca Aronde of course.
They also have more obvious stuff like American Graffiti too. (very obsessive study of all the cars in he back of scenes that you probaly never even noticed.)