When we were in Italy we noticed a lot of padlocks in public places. They're left on bridges, posts, fences, etc by lovers declaring their eternal commitment and, presumably, they 'throw away the key' after locking their love safely to a post.
Our guide told us this tradition came from an "Italian movie for teenagers" but Wikipedia tells me it originates in China. Either way it makes for cool photos. These two are from Flicker (top photo by EnDie1, bottom photo by Enzo D.).