chalk outline

A chalk outline, this one advertising M&M's has the familar outline of one of the M&M's character. Things like this attract people to them, as people always want to look when something bad happens or has happened. The chalk outline gets people to talk about it, even if people disagree with the method, it gets them talking about it. If this is applied to something more serious, it make shock people enough to help out a cause.
The second is an anti-smoking campaign, using the same sort of idea.