Passive Income: Summer 2014 edition

What is your source of passive income? You can find mine here: http://elonm.roon.io/20-8-2014-2

Earlier editions of this topic: * https://news.ycombinator.com/item?id=7094402 * https://news.ycombinator.com/item?id=6661536 * https://news.ycombinator.com/item?id=4639271 * https://news.ycombinator.com/item?id=387789 * https://news.ycombinator.com/item?id=5903868