Island ECN, owned by Datek Online, offers trading, mainly in Nasdaq-listed stocks, from 8 a.m. to 8 p.m. EST.
MarketXT, formerly Eclipse Trading, offers after-hours trading to retail clients of Morgan Stanley Dean Witter's (MWD) Morgan Stanley Dean Witter Online, Mellon Bank's (MEL) Dreyfus Brokerage Services and clients of Salomon Smith Barney. It is also available to clients of Mydiscountbroker.com, CyBerCorp and Interactive Brokers. Clients can trade 200 of the most actively traded New York Stock Exchange and Nasdaq Stock Market issues, 4:30 p.m. to 8 p.m. EST Monday through Thursday.
explains how the rules change when the sun goes down in Investing Basics: Night Owl, a section devoted to after-hours trading.