My first two-day hackathon I created an real-time multiplayer game called QuizTime. Users go to the site (http://qu.izti.me) and list items that relate to a set category. Guesses can only be picked once, so you have to be quick. The user with the most correct answers wins the round.
Find the source at my GitHub repo
This was not my first time making a socket.io game. I made the back end in Node.js and used Socket.io to handle the websockets. Ben Davis, who I met the first morning, created the front end using Twitter Bootstrap and CSS know-how.
And to keep things interesting, I created an Android app using PhoneGap that worked pretty well on tablets.
I got my fair share of swag and delicious food. In the end, some more thought out and useful hacks won. I'm excited to see where truBalance goes with their idea, it's very promising.