A short list of typical brokers that offer ECN access and the extended hours available is listed below. This list is meant to be illustrative, not exhaustive.
Ameritrade (via Island ECN)
Hours: 8am-8pm Eastern; limit orders only during extended hours.
E*Trade (via Archipelago ECN)
Hours: 8am-8pm Eastern; limit orders only during extended hours. Note that eextended-hours orders can be placed even during regular market hours; these orders may be filled during normal or extended-trading hours.
Fidelity (via Redibook)
Hours: 7:30-915am and 4:15-8:00pm EST; restrictions on order types.
Harris Direct (via Redibook ECN)
Hours: 8-9:15am and 4:15-7pm Eastern; limit orders only; round lots.
Schwab (via Redibook ECN)
Hours: 7:30-9:15am and 4:15-8pm Eastern, Monday - Friday; limit orders only.
TD Waterhouse (via ???)
Hours: 7:30-9:30am and 4:15-7:00pm EST