One of the oddest things I've seen since coming to Pittsburgh was a protractor superglued to the stone in my neighborhood. It turns out that there are hundreds of them cropping up, sealed to surfaces so securely that authorities intend to charge whoever is doing it with a felony. Each is penned with a unique number, and tracking them down is becoming a local mystery.
if you're interested in the protractors, do yourself a favor: Go look for them. In the process, you'll discover that we live in a special city, full of back alleys and secret passageways, theatrical vistas and hidden nooks, and anonymous people making beautiful things for everyone to enjoy — treasures that can only be enjoyed on foot (or on bike).
There's plenty more discussion threads on the 'net, for sleuths and curious locals to enjoy: Vannevar Bush wonders at the culprit; there's a lengthy discussion with photos at Bike Pittsburgh; and this Livejournal thread appears to be one of the earliest public WTFs. Here's a CBS story.