1) The new TerraLook dbase from USGS.
View Changes to the Earth’s Surface Over the Last 30 Years.
http://www.resourceshelf.com/2007/07/27/new-database-terralook-view-changes-to-the-earths-surface-over-the-last-30-years/

2) The new New USGS Map Locater (some fee-based maps). It’s actually a Google/USGS Mashup.
http://www.resourceshelf.com/2007/08/21/new-usgs-map-locater-and-downloader-delivers-topo-maps-online/

3) If you want to download any of the USGS imagery (here you can also find the specific dates the imagery were taken) go to:
http://seamless.usgs.gov/
All of the data is free.

4) Finally, from Trulia, Trulia Hindsight
“Trulia Hindsight, a new site that we’ve developed with design firm Stamen, provides an animated map of residential properties in the US. With Trulia Hindsight, you can search for specific cities, neighborhoods, or even streets and then watch as properties appear on the map in the year they were built. A slider control allows you to see when the majority of development occurred, pause on a specific time frame or focus on only homes built before or after specific years. Truly a unique and fun way to visualize the development of cities, towns or blocks . . . be careful you may spend hours cruising around the country.”
More here:
http://www.resourceshelf.com/2007/06/01/new-real-estate-info-database-trulia-hindsight/