Specifically for Science Hackday there's http://www.sciencehackday.com/ http://www.sciencehackday.com/. In general http://lanyrd.com/ http://lanyrd.com/ is the

Specifically for Science Hackday there's http://www.sciencehackday.com/ http://www.sciencehackday.com/. In general http://lanyrd.com/ http://lanyrd.com/ is the closest I've found, but it's still not comprehensive.