A love lock on the Pont des Arts in Paris. It's a wooden footbridge across the Seine. Couples place a lock on the bridge and toss the key into the Seine. Of course the city comes along every so often and cuts them off. But for a while, a symbol of your love is locked onto this bridge in one of the most romatic cities in the world.
I don't know when this started but we discovered them the last time we were there. Of course we didn't have a lock...but I vowed that the next time in Paris I would place a lock on this bridge.