Stuff made of iron or steel that at one point presumably had some kind of use, but has been left exposed to the elements and generally abandoned and mistreated. Baltimore, with its crumbling industrial base, is a good place to be if you like rusty junk. Fortunately, I do.
One fine example can be found along Falls Road in Baltimore. It's an old wheel near the Baltimore Streetcar Museum. It's approximately twelve feet across with eight spokes as thick as my leg. What's it doing there? What was it once part of? Now it's interesting rusty junk.
update 2/23/2001:I found out what that wheel is. The City Paper published a historical article on Baltimore's cable car system (only operational for about a decade at the end of the 19th century) illustrated with a photograph of that wheel. It seems that it was one of the "sheaves," the wheels that pull the cable of the cable cars.