I also recommend Ameritrade, I think they have the lowest commissions of the major internet brokers. I used to be with E*Trade and while I like their site design, the low frequency of trades that I do isn't worth the 19.99 commissions they charge. E*Trade has more research tools also, but I do all my research on other free sites.
I did hear about one alarming incident, although not first-hand. Someone I know at work apparantly was swindled out of tens of thousands of dollars from his Ameritrade account. From what I gather it was an inside job, not just an account password hack. I really want to talk to the guy in person to find out more about this.