Screen shot 2011 02 22 at 2.49.11 PM 220x209 A sweet Foursquare hack lets you know where you checked in one year ago each dayThis past weekend, Foursquare held its first “hack day” at New York City’s General Assembly, inviting developers to spend the day building new apps with its open API.

Explaining to my non-techy friends why I love Foursquare is hard sometimes. “You want to tell everyone where you are?” They ask, “Isn’t that dangerous?”

“Well, no, it’s not like that. There are discounts, tips, and the fact I can connect with friends nearby,” I answer as I compulsively pull up the app on my phone, attempting to nonchalantly glance at its screen, searching for my current venue. But one of the reasons I really love Foursquare is the fact that I have a personal collection of my habitual data, travels and now photos; however it’s also a feature I hardly ever used…until now.

One of Foursquare founder Dennis Crowley’s favorite projects from its Hack Day is 4squareand7yearsago, an app that pulls in user information from one year ago each day and sends it to you in an email. I haven’t received an email yet, but expect it to look something like this:

5466256286 5797e0afcf b 520x431 A sweet Foursquare hack lets you know where you checked in one year ago each day

Now if only I could use this information when filling out my tax returns…