Like the World Economic Forum in Davos, the conference brings together leaders from a variety of fields to brainstorm about political, cultural and economic advances. But Summit Series targets the rising generation: Its headline speakers included the creators of ventures likeDonorsChoose.org, Second Life, Blackboard, Threadless and Craigslist. They and some 650 acolytes came to Washington for the fifth and biggest gathering so far in a series that aims to inspire and train a new wave of innovators.