The premise of the show How I Met Your Mother is that in the year 2030 the narrator (Ted) is telling his kids how he met their mother. He starts the story 25 years earlier (i.e. in 2005), and thus far (after 6 years), we've gotten a lot of hints, but we haven't met the mother yet. However, it's quite apparent that Ted has in fact been at the same venue as the mother several times. In a hypothetical world where Ted and the mother use Foursquare, I thought it would be neat if they could compare checkin histories and see all the near-misses that they had over the years.
Intersquares does exactly that: you can sign in with your Foursquare account, and then once your checkin history is processed, another user can sign in with their account, and you'll both be told where you were together (whether you knew it at the time or not). This can be great for remembering first dates or for finding close calls.
This was my first hackathon, and I feel like it went pretty well (i.e. I managed to finish something). I definitely tried to keep the hack part in mind, as far as getting something together quickly. The code has a few dodgy technical decisions (keep all checkins in one entity property? what could go wrong?). It was also helpful to build on the same stack as Stream Spigot (App Engine, Python, Closure, Django Templates) so that I could lift a lot of utility code and patterns. Next time, I'd like to be a part of a team though: while being a one man band is satisfying (check out that logo), it does tend to limit the scope to toy-like apps such as this one.
The hackathon has prizes, so if Intersquares intrigues you, your vote is appreciated. Definitely check out some of the other entries too. I haven't gone through all of them yet, but so far Near, Magic Muggle Clock and Plan Your Next Trip all seem really neat.
Update on 9/28/2011: Intersquares was a finalist in the hackathon!