They have/had this, but it hasn't proven terribly popular: https://contributor.google.com/ https://contributor.google.com/ edit: I say "had" because I think it

They have/had this, but it hasn't proven terribly popular: https://contributor.google.com/ https://contributor.google.com/ edit: I say "had" because I think it has changed forms over its lifetime. I think originally you could simply load up credits and it would cover ads across the web with blank images.