I will probably get torn down by the HN crowd for this one, but I think the best one around is still the TechCrunch hosted one annually at Disrupt. If you've n

I will probably get torn down by the HN crowd for this one, but I think the best one around is still the TechCrunch hosted one annually at Disrupt. If you've never been to one that's a great starting point, and it's free - I am a skeptic of the new trend of for-profit hackathons with entry fees, personally. I still think that's a good starting point. The Startup Digest is a good source of information on upcoming Hackathons.