Arby's Hijacks Billboards with Thought Balloons

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

Billboard With a Hanging Man

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."
-- Ananova

McBillboard: Starbucks Takes It On the Chin

McDonald's has erected a billboard in sight of Starbucks headquarters in Seattle declaring, "four bucks is dumb." -- full story

Painting iPods

Painters create iPod nano wall mural ad.
-- top left pixel