Not sure if anyone's posted this before but:
After a round of Oblivion last night I checked out the marketplace to download the PGR3 updates. Looking through the list I noticed the "NEW!" text next to Burnout Revenge (I know NEW is a relative term and can mean weeks in XBL terms), So I checked it out and they had a whole bunch of new cars for download, all free.
The cars were each sponsored by a different company
(I think that's all of them)
Not sure how good any of them are as I haven't tried to use any of them yet, but hey, free cars.