What is the cheapest way to trade stocks?

There are a variety of options and services for trading stocks. A range of services online offer stock trading for as cheap as $4.00 to $4.50. Some of these low-cost services, however, pay for the cost of the trades by giving sub-optimal pricing; specifically, by increasing the buy/sell pricing spread. Some such services may also scrimp on other important features such as a toll-free customer service phone number, and advanced trading features.