Merrill Edge is a full-service, beginner-friendly broker with low fees, but isn't great for expert traders.
TD Ameritrade is a strong trading platform with services that are worth the extra cost.
For the lowest fees offered by a reputable online broker, OptionsHouse is going to be your best bet.
E*Trade is user-friendly with great pricing, and has a robust library of investment information.
Scottrade provides consistant, quality customer service, and a user-friendly platform at a reasonable cost