Agenda Inc. has a service they call American Brandstand which keeps track of any mention of a brand name in the Top 20 singles chart. Oddly enough, Pepsi was mentioned more than Coke, at least in 2004. Automobiles get mentioned quite a lot as well, which seems obvious, since songs about cars go clear back to the early days of rock and roll. I'm not sure if the mere mention of a product in a song does much for a company, but I guess any exposure is good exposure.