I haven't been to that many, but of the hackathons I've been to, I think my favorite was Abe's 'come at me bro' which used twitter and some prediction service to find insults to sling at people.
What are some of your favorites?
Hack Princeton, a couple of guys spent the two days sleeplessly soldering together a small 9x9x9 cube of blue LEDS. They hooked them up to an arduino, a couple of controllers, and demoed a game of 3D pong.
Devon Peticolas added Gerard O'Neill.
I really liked PoachBase, which used CrunchBase to determine which startups were likely to "die," then analyzed the GitHub profiles of developers who worked at those companies to give you suggestions on who you should try to hire.
Andrew Flockhart added Tal Safran.
Yo yo, Andres 4 president.
Andrew Flockhart added Andres Campanella and Misha Ponizil.
I really liked that tumblr hack where the guy went like "and so naughty" from the last hackNY hackathon.
There will always be a special place in my heart for Locreep.
Also, the Twilio battle tanks game that Joey helped make. It was surprisingly responsive, thus enabling me to whoop ass.
