E-tradre Schwab, TD Ameritrade, and now even Interactive Brokers offer free trading on US stocks and ETFs on their platforms. Some even offer free trading on Canadian market.
While trades are free, the brokers still earn money on you. Most of them borrow your shares to short sellers which pay a loan for them to cover shorting activity.(shorting involves selling a stock you don’t own and buying it later). But big dirty secret is in Data. These brokerage houses will sell your orderflow; data containing trading information to high frequency traders (HFTs). HFTs have faster access to stock exchanges and can front run your orders bidding price up before your order arrives.
Michael Lewis has written a book about it. The Flash Boys which is now coming as a film later. This is short intro to the topic.
My trades are very low already with Interactive Brokers($0.3 – $0.5) And I am not willing to sell my data to wolves that chap.