the first time i actually took notice of them was back in 2004. my friends and i were walking back to our hotel after a few drinks, and i let out a loud, long gasp. they jumped and grabbed each other because they thought i'd seen a cockroach or something. but nope. it was just a painted manhole cover. one very much like the one at the bottom right of this little set.
the first one is at a fire engine zone at narita airport. the two cherry blossom ones are just from walking around tokyo. the wavy castle one is of osaka castle, as is the painted one below it. the bottom left is also of osaka found along dotonbori-dori. for a closeup, just click on the picture.