What is the best online stock trading site?

I want to trade penny stocks and a few regular stocks. I don't have a lot of money to spend paying the site, but I would like to at least have a feature where if a stock lowers to a certain point it will automatically sell if that's available. Other than that I just want inexpensive service.|||You can set a stop order (rather than limit order). That will trigger the selling when the price dips below. If you don't mind paying extra $$$, you can buy or search a free automatic trading program online to do the job for you too!