Robinhood is about to give Coinbase, the cryptocurrency brokerage, a run for its money. In February, Robinhood, an online stock trading app, introduced a service that lets users buy and sell cryptocurrencies like bitcoin and ethereum without any added transaction fees. By comparison, Coinbase charges anywhere from 1.4 percent to 4 percent to buy and sell bitcoin and its ilk. Robinhood will also let users track price and news and set up alerts for bitcoin and a dozen or so other cryptocurrencies.

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The Palo Alto-based Robinhood entered the crowded trading arena in 2013. Through its mobile app it lets users buy and sell U.S. stocks and ETFs without any trading fees. The platform is especially attractive to less-wealthy millennials who are interested in investing but can't afford to spend $7 to $10 per trade with firms like E-Trade or TD Ameritrade. For users that can afford a little extra service, the company launched Robinhood Gold, a premium paid service for experienced investors that offers margin and extended-hours trading. It's priced at $10 per month.

Co-founders Baiju Bhatt and Vlad Tenev were former classmates at Stanford University. Before launching the company, they ran two other firms selling trading software to hedge funds. Robinhood doesn't have any brick-and-mortar locations — one of the main reasons why it's able to charge a zero commission. Over the past year the company doubled the number of its users to 4 million (surpassing rival E-Trade). It closed a series D round in May, raising an additional $363 million and giving the company a value of $5.6 billion.