Source of Passive Income? (2015 edition)

What is your source of passive income? Context: http://goo.gl/0zjoKp

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