It might be a horrifying prospect to invite a bunch of people with the skills required to discombobulate your beloved widget to come and do it before your eyes. After all, they might build things that you don't like, or aren't consistent with your brand. But knowing what your product isn't is more helpful than it seems. A long, unguided hack session pushes new ideas to their logical limits and helps define which ideas deserve core in-house attention. It also helps determine which tweaks can be left for third parties or competitors to try. (Left, a hacker makes last-minute changes to her project.)

It's not enough to give people a product and let them use it in some arbitrary way; these days, people want to benefit from their product interactions by seeing them in aggregate. A group of New York developers created Foursquare and 7 Years Ago, a service that emails you with your whereabouts and activities of exactly one year ago (assuming you were using Foursquare back then.)

A group of hackers led by New York developers Aidan Feldman and Brian Fountain spent their day building something they're calling Voxora, which lets you leave voice notes at certain places for friends to find when they check in. "We're going to take pains not to tell people how to use it," Fountain said. "It is what you make it."

One popular hack called "Hail to the Mayor" allows you to associate your Foursquare account with a few favorite songs, which would then queue up on a bar's jukebox as soon as you walk in--if you're the mayor of that establishment. Sure, it'll require more than one day's work to make this hack a real working service, but damn will it be cool.

Unless you're Google, your algorithms probably suck compared to the abilities of even the most average dude. A brilliant hack called Moonwalk allows Foursquare users to broadcast questions in real-time to every one else in the venue, like Quora for the people around you.

A few months ago, we covered Nick and Erin Sparling, two highly-skilled hackers (and brothers) in Brooklyn who decided they wanted to use a Foursquare check-in, not a key, to get into their apartment building. Thanks to an enormous response from readers all over the Web, the Foursquare Door is now a fledgling product; you can see DIY instructions at foursquaredoor.com. As it turns out, people like when your technology improves the physical world, too.

A bold hack called FourPlay got groans during the end-of-day presentations, but the idea is sound: It lets anyone on Foursquare find a date nearby after trading pictures. Yes, more and more people use the Internet for sex, and in the future, it might be all they use. Innovate for next year's tastes: some hot-and-bothered tech lover will need you.

One of the most ingenious things about a hackathon: It brings potential competitors into the fold. All the developers in attendance might have gone off to form their own location service with their own special features, but by making their platform available for all to play with, Foursquare has pre-empted a bunch of unrealized competitors and made them assets. To see the rest of the hacks and infoformation about the developers that built them, check out Foursquare's hack day Wiki here.