Timehop is a fun service that indexes all of your photos, then displays historical photos that you took on this day in history, its tagline sums it up nicely: Timehop helps you celebrate the best moments of the past with your friends.
Start by downloading the iOS app (free, App Store) to your iPhone, then install the desktop sync client on your Mac or PC. The app is exclusive to iOS for the moment, but the desktop client works on both OS X and Windows and if your don't have an iPhone, you can view your hops on a Web interface.
When you open the app it prompts you for your Facebook credentials (or uses SSO on iOS 6 or higher). From there you can connect it to many of the other popular photo sharing services including Twitter, Instagram, Foursquare, Dropbox and Flickr (pictured). You then connect it to your desktop photo libraries via the desktop sync client.
I was able to add all my photo accounts via the iPhone app, except Dropbox which gave me several "application errors." I had no trouble adding my Dropbox > Camera Uploads photo via the desktop sync client for OS X though.
Once you have it all set up you're able to "hop" through years of photos on your iPhone and it's a blast. The interface shows your photos (and Tweets) from this day in history on all the previous years that you have data for. All photos are kept private and secure and only shared with you.