Here's a couple photos of slag from the north end of Eureka, Nevada to go along with Maria's post and some great photomicrographs of slag here and here. There's another slag pile from a different smelter on the south end of town, which looks very similar. The slag piles were formed from the smelting of the dominantly lead-silver ores of early Eureka days - smelters were active starting in 1869. Much more recently, starting sometime in the 1970's, gold has been produced from several mines in and near Eureka, and molybdenum was discovered by ASARCO at nearby Mt. Hope in the 1970's and may go into production by 2010.
UPDATE: Antimonite has recently posted a photo of iron slag from 700 A.D.