When I drive past a house where dogs are left outside 24/7 I send them a post card I bought from Dogs Deserve Better.

http://www.dogsdeservebetter.com/home.html

I send it to them in the mail (with no return address) because no dog deserves to live it's life alone.

edit:
This is one style of postcards I bought to send to houses that have dogs left outside all year long.
http://www.cafepress.com/dogsdeserve.8539313