Over 10 years ago, there used to be a brokerage called "Brokerage America" which was 0 commissions. I had an account with them back then and I loved it. They didn't last very long and I don't remember if they got bought out or simply went out of business.
Now there is Robinhood.com, which is also going to be commission-free on US Stocks and ETFs, but they haven't really launched yet and you have to put yourself on a waiting list. I got on it about a year ago and I am like # 234,000 with about that many after me (you can check how you are progressing on the list on their site). Their platform will be only on smartphone apps, not on a website. Right now they have only come out with an app for iPhone and they promise that they'll have an app for Android in 2015. I wish they'd get on with it.