I once pitched the idea of having e*trade write "other things to do with your money" on one dollar bills. But my otherwise brilliant CD shot it down for legal reasons. So I turned the idea into an art project called Post No Bills.
I was pleased to learn from Chimp Media Monitoring that a brand finally did something similar that's truly great.
Saatchi & Saatchi scattered £1,500 in cash on the ground in a public square with this sticker for Carlsberg. The PR it generated was mentioned in this story on experiential branding by Claire Beale in The Independent.