I think that the use of bumper stickers has diminished due to people leasing cars these days instead of owning them outright. I notice that university towns tend to have more vehicles with bumper stickers. One that I saw frequently in the Kalamazoo area was "Free Leonard Peltier", Kalamazoo being an area with a large indigineous population.
http://www.freepeltier.org/
One that I particularly liked was "What if the Hokey Pokey IS what it's all about?"