Trading - may it be currencies(forex), stock market or commodities (gold, oil, corn, etc).
But of course, you need to learn about it to be able to earn passive income from it. READ: high risk
I just found out about a site where you can do actual practice trading without paying anything. It's actually a game introducing trading to newbies and if you do good, they convert points to $$$ payable via paypal/skrill.
Register here:
https://bitly.com/lrn2trade
You can also check intro video here:
https://www.youtube.com/watch?v=WtwM3gwLCXs