Timehop is a service that sends a daily email to you of what you did one year ago - so long as you did something on Facebook, Twitter, Foursquare or Instagram.
If you did nothing, no email.
My only quibble is that you must have a Facebook account to sign in. I know plenty of people who either don't have a Facebook account or won't use it to login somewhere for fear of giving away everything they've ever done on Facebook to some random web app.
Also, Twitter only allows access to your last 3,200 tweets. If, like me, you've tooted more often than that in the past 365 days you won't see anything from your tweets. Timehop is apparently developing a work around.