Looking Into The Past
Jason Powell created a brilliant series of images juxtaposing vintage Library of Congress images with their modern day counterparts. It works well with buildings but even better with people.
The idea spread into a full-fledged Flickr Group and now loads of people are putting their own take on the concept up. Lots of quality submissions in there but people always seem to make for the most effective subjects.
see link above image for full photo set