Most new ECNs, such as Island, only let customers enter limit orders. That means they will only accept orders to buy and sell at a specific price. If a trade can't be consummated at that price, the order will go unfilled. Essentially, the systems simply list orders to buy and sell, and when they detect a match, the system automatically executes the trade. Instinet, which has high trading volume because of its institutional customers, allows market orders as well as limit orders.
Market orders let customers buy or sell at the prevailing market price, whatever it is.
Also, most ECNs only trade Nasdaq stocks. Instinet trades Nasdaq and New York Stock Exchange shares. That depends on your brokerage firm. So far, it appears that most online brokers are charging the same commissions for trades before, during and after traditional trading hours. But it doesn't get prices from all trading systems.