(Video Link)
Designer Fabian Brunsing created "Pay & Sit: The Private Bench". Normally, it's covered with spikes. But if you slip half a Euro in the coin slot, they retract for a fixed period of time. An alarm goes off a few seconds before the user's time is up and the spikes rise.
via Geekosystem
http://www.makita.com/en-us/Modules/Tools/ToolDetails.aspx?ID=10948
Vive la résistance!