Historypin lets users search, view, comment, or contribute to a Google map with overlaying vintage media - photos, audio, and video. This is particularly cool when you browse to the street view mode and can shift back and forth between the historical image and the street view image from today.
(For example, you can explore images of Red Square from different eras, or look at a street corner in London before and after the Blitz, or see early images of the Brooklyn Bridge superimposed on a contemporary skyline.)
Pinned sites are international, with 55,966 contributions to date, and cover a timeline from the 1840s to the present.
There are a handful of Pittsburgh images, but not much yet.