Arby's (a fast-food chain) flies balloons with its logo in front of others' billboards in Atlanta. The chain's current slogan is "I'm thinking Arby's", naturally.
-- ads of the world
Chinese samaritans were outraged when they saw this billboard for a local mall featuring a hanging body and copy "It's better to invest money here than put it into the stock market."
McDonald's has erected a billboard in sight of Starbucks headquarters in Seattle declaring, "four bucks is dumb." -- full story