Building a smartphone or tablet app requires a fair amount of technical expertise, but it can pay off handsomely if successful. It doesn't have to be particularly complex or chock-full of features either; a simple one that solves a problem or is entertaining enough to attract users works fine. You never know what may suddenly become popular. For example, the minimalist game Flappy Bird was such a craze that at one point its developer was earning $50,000 dollars a day through in-app advertising. Of course, expecting a success like that is foolhardy, but one can always hope.
Portfolio income can come from multiple sources – interest/bond coupons, stock dividends, financial strategies including derivatives and capital growth.  Each offers some cash flow and some also offer potential capital gain with some risk from liquidity and volatile prices.  In this low-yield environment, many investors rely too heavily on cash flows and to pursue this are venturing further into riskier areas than they’d normally consider.