Meow Meow Meow Meow on My Cell
Check out this campaign done by http://www.mfoundry.com/ for Purina. It's a good example of free, branded content. The barking dogs ring tone scared my cats. The meowing cat ring tone made them run searching for the unknown cat. Fun campaign.
On the down side, it suffers from the challenges of all mobile content developers – only works on a limited number of cell phones and none of the handful I own.