Several interesting sites or personal pages about the aesthetic and cultural aspect of manhole covers :  

Ironwork was a basic part of soviet culture. This russian group intend to show most manhole covers of the world... Many patterns to see yet there.

Artworks of Bobby Mastrangelo with manhole covers patterns.

Dan, as fine art professional photographer travels the world. Manhole covers are for him signs to identify places...

Dave's manhole covers page

Dave too, likes manhole covers !...

Manhole covers of Tokyo

Pictures from Greg Wilmot

Manhole tops in Sendai City

Asian manhole covers

manholecovers.net

A home page of various manhole covers pictures.

This site shows the underground face of urban environment.

Very nice site with Flash , to discover the underground of Paris and its surroundings and to get many usefull informations to visit it.

With its large database, this is a reference site for anyone who wants to know the strange part of our urban and industrial world and live in to.