Others offer high-speed fast order routing. Still others route both NYSE and Nasdaq trades. Stiffler has studied ECNs extensively and maintains that some offer options that others don't. To access an ECN, you need to sign on with a broker that's partnered up with at least one of them. Active traders love ECNs because they can send out limit orders that lie within the bid - ask spread market makers maintain.
Many traders live and die by such fractional gains.
Individual traders aren't the only ones who use ECNs. Many institutions route their orders through them as well. To make things even more complicated, each of these ECNs handles orders in a slightly different manner. And all charge different commissions, which tend to be a fraction of a penny or more per share. For an active trader you don't need to pay attention to those other ones.