One of my primary sources is from staking bounty amounts.  It is small from any individual bounty reward, but over time it can add up to a lot.  Particularly when you consider the effects of compounding, it could serve to put you over the top in terms of wealth accumulation.  Remember, FV = PV * [1 + (i * n)], where i is the assumed growth rate, PV is your starting principle, and n is the number of periods of the growth rate.