Google Street View is a fantastic way to virtually explore the world, but until today, you could only see the latest imagery available. That kind of sucks if you had any interest in how a place has changed over the years. Google Earth has allowed us to check out old satellite images for a while now, but that's only good of you want to see what buildings have come and gone. As of today, Google has opening up this same type of access to historical imagery on Google Street View, so you can see the changes from an up close and personal angle.
The big difference is that while Google Earth uses satellite and aerial images from several sources, Street View relies on Google's own fleet of cars, trains, and even backpack wearing adventurers to document all of the nooks and crannies of the world. This means that their street level imagery only extends back to 2007, but even in that relatively short time, there have been some pretty remarkable changes.
To see if an area you're exploring has historical Street Views, look in the black box in the upper left corner of a Street View window for a clock. Click on it, and you'll get a slider that lets you look at the older street views in a window over the most recent image. Then when you see what you want, you click on the window to see the historical image on the full screen. Even with just seven years worth of views this seems to be a time waster par excellence, so imagine what it might be like in another 20 or 30 years if Google keeps this up.
For example, here in New York one of the biggest changes in recent years has been the construction of One World Trade Center, aka The Freedom Tower. Now you can chart its progress using Street View from just getting off the ground in 2009, to its full height in 2011.
Seven years may not sound like a long time, but if you check out your own area, I think you'll be surprised how much has changed without you even noticing.