You've probably seen their commercials running on CNBC. Scottrade used to be a no-frills online brokerage that offered $7 per trade; basically you got what you paid for. However, the company has evolved and now features considerable services to its clients. It's a great, reputable service for new investors.
Ameritrade is another great service because they have low commissions ($10.95). Best of all, the quoted commission price includes limit orders. If reputation and ease of use is what you're looking for, consider Ameritrade.com.
Sharebuilder is hands-down our top online broker for new investors with not a whole lot of money to get started. With their low commissions, no minimums, and tons of investment options, we seriously think you should give them some thought for opening an investing account.
BrownCo is for the more experienced investor. Because you're more sophisticated and have a higher account balance, BrownCo rewards you with great services at low prices. Their commissions are as low as $5 per trade.