Here's a clever ad that UNICEF put out a few years ago to raise funds for people who have been maimed by landmines. Stickers, camouflaged to look like pavement, were left on sidewalks. When someone steps on one, it sticks to the bottom of his/her shoe. The underside informs the victim that if it had been a landmine, s/he would have been mutilated.
Link via Everyday, No Days Off | Image: Leo Burnett
And if you actually read the ad, it said that the stickers were placed in front of donation booths, and that their donations rate increased... so, not really a waste of money... (and even if it didn't increase their donations, at least they're informing otherwise oblivious people about the tragedies that happen in many countries...)