Service companies can send out their messages very inexpensively with a little creativity, and improve the landscape to boot.  The above truck belongs to a recycling company.

The Plumbing Contractor above found a sure-fire way to get attention and new customers.  And how do you like this – from a book store chain:

Makes me want to go out and paint my truck.  But it’s still covered with yellow pollen, so I can just write my message with my finger.