![manhole advertising](http://photos1.blogger.com/blogger/353/70/320/makrolon_manhole_advertising.jpg)
Haven't witnessed it in action myself, but Bayer is making some special plastic material called Makrolon which can be successfully used to cover urban manholes with advertising messages.
Also, here's a rug that looks like manhole cover, a collection of Japanese manhole cover designs, and a story about a town of Vail that sells branded 52-pound covers as souvenirs.