PhilaPlace is an interesting project:
PhilaPlace is an interactive Web site, created by the Historical Society of Pennsylvania, that connects stories to places across time in Philadelphia’s neighborhoods. PhilaPlace weaves stories shared by ordinary people of all backgrounds with historical records to present an interpretive picture of the rich history, culture, and architecture of our neighborhoods, past and present. The PhilaPlace Web site uses a multimedia format – including text, pictures, audio and video clips, and podcasts – and allows visitors to map their own stories in place and time. More than a Web site, PhilaPlace includes ongoing community programs and publications, from workshops for teachers, to trolley tours, and exhibits. PhilaPlace is an engaging, meaningful way to understand more about where we live, and will serve as an enduring record of our heritage.
Another interesting project is at LookBackMaps:
These folks have built an iPhone app that enables a person to stand at a street corner (say), and call up an historic image taken from that precise location, to have a "then-&-now" comparison.
Lookbackmaps is a simple, yet robust way of visually organizing, exploring and engaging in history and historical photographs through web and mobile-based maps.
Through the online mapping of high-resolution public photo collections and geotagging technology, Lookbackmaps creates collaborative, standardized views into the past.
We're in the initial stages of planning to implement some features such as these into the Oregon Encyclopedia.