Named after the legend who robbed from the rich to give to the poor, Robinhood offers commission-free stock and option trades at a time when even the least-expensive brokers charge $5 per trade. That pitch has helped Robinhood open more than 4 million accounts since 2013, making it bigger than E*Trade, which went online in 1991!But where Robinhood can save users real money on commissions, the service trades user experience for tax inefficiency.