PayPal BattleHack Global Multi City Hackathon April 5-6 in SF (free, as in free to register, free beer + excellent local food + fun giveaways)
Chance to win $100,000 Grand Prize at World Finals for Best PayPal API integration etc.
Details at: 2014.battlehack.org (https://2014.battlehack.org/)
Video overview: youtube.com/watch?v=8oG74vkSyvY (http://www.youtube.com/watch?v=8oG74vkSyvY)
Register for San Francisco: https://2014.battlehack.org/san_francisco#register