Without investment it is impossible to earn ETH, it is possible to earn ETH through bounties. Bounties are the best way to earn some token my promoting their campaign and we can easily convert them to ETH.
Yes, participating bounties will help us to have ETH without any amount of money invested but it still need our time and efforts for these in order to receive tokens which convertible into either ETH or BTC. A lot of people doing these( including me) and we become more profitable if the bounties pays as what they've promise to us.