This app by Tim Varney of Troy Web Consulting first came to my attention with an article on All Over Albany (a site which you should absolutely check daily). At the time, I had not yet upgraded to a smartphone capable of downloading apps. However, with a very recent upgrade to an Android phone, I was extremely eager to try Tim Varney's Albany: Then & Now.
The app, which is a free download for both Android and iPhone/iPad, gives a terrific glimpse into the city's past by matching present-day map locations with century-old photographs of the exact same place. Each antique photo is accompanied by a brief, but excellent description.