A rash of low-cost, online-only financial advisory companies have hit the scene in recent years, threatening to steal customers from traditional advisors while also broadening the market for financial planning and wealth management.

Many of them are so-called robo-advisors which use computer models to recommended a portfolio of low-cost exchange-traded funds and mutual funds that match an individual’s financial profile.

These platforms have become so popular that even big Wall Street firms and discount brokerages such as Morgan Stanley, Charles Schwab and others have begun offering them to some of their clients. Fees generally range from about 0.5 percent of assets or less, which is about half of what traditional advisors charge.One of the larger robo-advisors is Wealthfront, which has more than $10 billion in assets under management and bills its service as financial planning for millennials. The company takes dead aim at the notion that clients need a “licensed expert” to help them make good financial decisions. And it promises to “build a free financial plan for the life you want and automate your investments at a low cost.”

Wealthfront’s investment principal is based on the idea popularized by Vanguard founder John Bogle that investors do better over time by buying low-cost, market-tracking passive funds than by hiring expensive portfolio managers whose active management approach often fails to beat their bogeys. The company calls its investment approach passive investing on autopilot.

Wealthfront charges a 0.25 percent annual fee on the amount invested. The company says its computer models have been built by a team of PhDs who implement time-tested investment strategies and develop a data-driven planning experience that’s personalized for each client. Moreover, it’s so sure of its formula that it says clients need never make a phone call to the company.

“By implementing a completely software-based solution, informed by decades of academic research, Wealthfront is able to deliver its automated investment management service at much lower cost than traditional investment management services,” the company says.

WiseBanyan follows the same passive ETF portfolio strategy as Wealthfront but goes even further down the price scale—all the way to free. It bills itself as “the world’s first free financial advisor.”

Instead of charging for investment management, the company says it earns money solely from à la carte products and services designed to provide personalized value to our clients. Those services include tax protection and Portfolio Plus, which enables  clients to design their own portfolio using an expanded list of investments across a variety of asset classes and sectors.

Facet Wealth, meanwhile, is a hybrid robo-advisor and traditional financial planning firm with a focus more on the human side—albeit online. It’s also different from Wealthfront and WiseBanyan in that it doesn’t market itself directly to consumers. Rather, it looks to buy smaller accounts from other advisory firms that may be unprofitable for them and not getting as much attention as the client would like.