Do you know your neighbors? If you have their contact information-- give them a call.
I'm guessing you don't live in a neighborhood with a HOA? If you do, that would be my first step. They can give notice to the owners citing cc&r's with a time limitation to have the porperty cleaned up. If they do not abide, the HOA can have the yard maintained and put a lien on the home.
My next step would be to report it to the city or county, whomever you pay your property taxes to.