Throughout the history of the stock market, sales commissions have weighed on investors' results by skimming money off the top that could otherwise go toward buying shares of profitable stocks. Yet over the years, commissions have gotten a lot less onerous, as the rise of discount brokerage companies reduced what had been hundreds or even thousands of dollars going toward commissions and replaced them with deals that could often result in trades costing less than $10. Those smaller amounts made it possible even for small investors with modest savings to invest in the stock market.

Some brokerage companies have even offered commission-free trading to their clients. With some brokers, there are restrictions on the number of trades you can do, while others limit the types of investments that qualify for commission-free treatment. The question investors need to ask is whether commission-free deals are actually good for them, or whether the brokers in question intend to make up their lost commission revenue in other ways.

The history of commission-free trading
Unfortunately, many past commission-free trading deals have disappeared over the years. Wells Fargo's (NYSE:WFC) WellsTrade service, for example, used to offer 100 commission-free online trades per year, but as of April 2013, the bank took away that feature for newly linked brokerage accounts. Some smaller companies that used to offer free commissions have gone out of business entirely.

Source: Flickr user Rafael Matsunaga.

Yet there are still some offers out there. Bank of America's (NYSE:BAC) Merrill Edge, for instance, allows customers with $25,000 or more in combined bank or brokerage account cash balances to get 30 free trades each month. For those who don't have that much to invest, newer upstart companies like Robinhood and Loyal3 offer commission-free trades as well.

It's important, though, to look at the fine print before you pick a service. With Robinhood, for instance, trading has to happen by mobile device, and the no-commission treatment only applies to U.S.-listed and over-the-counter securities. Listed foreign securities have a $50 trade commission, according to its March 2015 fee schedule. Moreover, if you need broker assistance over the phone, you can expect a $10 commission. Similarly, Loyal3 has a limited array of stocks it offers.

Exchange-traded funds and commissions
If you're not dead-set on trading individual stocks, then your commission-free offerings expand greatly. That's because a wide variety of brokers offer ETFs at no commission.

Some brokers have taken the opportunity to offer their own proprietary ETFs at no commission. For instance, Vanguard and Schwab (NYSE:SCHW) are among the companies that have built up extensive ETF portfolios and encouraged their customers to use them by offering commission-free purchases and sales.

Meanwhile, other brokers have gone outside their ranks to build partnership deals with third-party ETF providers. For instance, Fidelity has a deal with BlackRock's (NYSE:BLK) iShares to offer ETFs at no commission, while TD Ameritrade (NASDAQ:AMTD) offers more than 100 different ETFs from various providers, and E*Trade Financial (NASDAQ:ETFC) has a similar offering of third-party ETFs.

What's in it for them?
Of course, free commissions only make sense to brokers if they help generate business elsewhere. For Bank of America, the hope is that you'll have a big enough overall financial relationship with the bank to justify cheap commissions. Similarly, brokerage companies offering ETFs hope that you'll also do some commission-generating stock trading with them, and they can also benefit from the management fees ETFs charge on an ongoing basis.

The bigger question you should ask is whether free commissions are something you should take maximum advantage of. Remember that you don't have to use all of your free trades, and long-term investors probably shouldn't, especially if it would change your strategy to be more focused on trading in and out of stocks on a frequent basis.

Finally, make sure you don't assume that a commission-free arrangement will go on forever. Especially with smaller companies, business models don't always work out, and that can lead to the hassle of having to move assets back and forth repeatedly.

Competition in the discount brokerage arena has been a positive for investors. By making sure you understand the reasons behind a broker's decision to offer commission-free trading, you'll be best able to take advantage of the cost savings without getting snared by unanticipated consequences.