When I was in library school we were asked to create a database that could adequately describe a visual collection. I had suggested that our test collection be a collection of manhole covers.
Snuggles, my partner on the project, insisted that we use a selection of Walker Evans' Farm Services Administration photographs. This was a more complicated choice and we ultimately learned more for that reason.
But I like manhole covers.
Here are a few from Japan:
No, this is not the same manhole cover. Please look closely :)
To see everybody else's photos, visit the mash-up here.