As an alumnus of the University of Louisville I follow the University's posts on flickr. To my surprise UofL posted some photos today from their archives and something that mentioned I site I hadn't stumbled on before. HistoryPin.
I spent some time playing around on historypin.com and it's pretty neat. Particularly when you overlay images against google street view images. You can even toggle transparency. The effect isn't always prefect as the streetview shots are rarely taken from the same angle (or with the same type of lens), but it's neat nonetheless.
Here's a screenshot I took of my browser showing another image from UofL's archives showing the Church of Our Lady during the 1937 flood (this one has a just a bit of transparency):