Ameritrade
Td ameritrade offers online trading services: broker services including online stock trading, online investing and option trading to help you reach your investment goals.
http://www.ameritrade.com/
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http://www.etrade.com/
Scottrade
Scottrade discount brokers offer investment services such as stocks, options, mutual funds, treasury bonds & retirement accounts. flat commissions as low as $7 per trade for online trading accounts.
http://www.scottrade.com/