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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.

Here's one happening worldwide next month: http://spaceappschallenge.org/

There are multiple hackathons depending on where you live (some are online also) and what you are interested in. Some that are interesting and in many cities include NodeCopter (node.js and drones) and the startup related ones - AngelHack and Startup Weekend.

A great site is listing many hackathons is: http://www.hackathon.io/

Try tracking the hackathon tag on Lanyrd and looking for hackathon groups on Meetup.com in your area :)

Also check out http://localhackathons.com/.