I traveled to San Francisco earlier this month to run the 100th Bay to Breakers 12K, so while I was making personal history I decided to brush up on my San Francisco history, too. While wandering the hilly streets of the Bay City, I equipped my iPhone with app Time Shutter — San Francisco Deluxe (it's also available for iPod Touch), which provides hundreds of historic city images and their present-day counterparts.
My hotel was on Union Square — a perfect jumping-off point for the more than 240 images provided in Time Shutter. Within the app, users are treated to a slideshow (it will run automatically or at your swiping speed, whichever your preference) of historical, fully captioned images of San Francisco landmarks. Immediately after each rendering, you'll see the present version of the location. Double tapping the screen will transition the images so you can quickly morph between the present and past.
Many of the included sites have been preserved, so the Flood Building looks similar to its 1908 version (minus the Gap, of course). Other sites didn't make it through history as successfully, though — the Orpheum Theatre-turned-parking-garage was a bit depressing. The app developers clearly made an effort take present-day photos as close to the angle and perspective as shown in the historic rendering. All of the locations included in Time Shutter are also denoted (and numbered) on a geo-coded map, which I found useful to create my own walking tour.
You don't have to be within city limits to enjoy Time Shutter — San Francisco Deluxe, but if you are standing on, say, Geary between Powell and Stockton streets, you can snap your own image of Union Square to create your own photo album. An overlay of the rendering will appear in your viewer so you can match it with your current view as closely as possible.
Time Shutter — San Francisco Deluxe is $1.99, but a free version of the app (with 100 fewer photos) is also available. I found the app to me more than worth its price, and I hope the developers consider launching the product in other cities.