Now available for Google Maps for Desktop.
From the Google Lat/Long Blog:
We’ve gathered historical imagery from past Street View collections dating back to 2007 to create this digital time capsule of the world.
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This new feature can also serve as a digital timeline of recent history, like the reconstruction after the devastating 2011 earthquake and tsunami inOnagawa, Japan. You can even experience different seasons and see what it would be like to cruise Italian roadways in both summer and winter.

The Hourglass Icon in the Upper Right Corner Shows if Historical Imagery is Available. Source: Google
From the WSJ
The time machine will be available in almost every location where Street View is in operation. For major metro areas, there will be 20 or more “time slices” to check out, while for most locations, there will be two or three, said [Luc] Vincent, [Google Maps Street View director of engineering].
Before this massive update, about 6 million miles worth of Street View imagery was available. As of today, including the time machine feature, you can find about 12 million miles worth of sidewalk-level, interactive photos to explore.