Check out TradeStation, Interactivebrokers, Schwab, Cybertrader, Fidelity, Pristine, MBTrading, MyTrack, TerraNova.
Each has something different to offer, depending on what level you are trading, how active you are, and what is most important to you, i.e. commissions (flat fee vs. /share)? trading platform (integrated w/charts, news, scans, etc.)? Trading rules vary also from company to company, some applying the Pattern Day Trader rules to IRAs as well as margin accounts.
Good luck!
Newly