Not sure if this is culture-jamming or an ad campaign, but this billboard shot by Giant Robot's Eric Nakamura shows a billboard hidden behind a grass-printed construction barrier. The board, aptly, reads "a little better."
In the "much better" category is this guerrilla gardening project by Netherlands-based interventionist Helmut Smits. He strategically plants trees so when they grow up they obstruct views of advertisements. (Via Guerrilla Innovation.)