Sharebuilder also has huge extra charges for penny stocks. It's not the bargain that it seems.
I would recommend either Etrade, Fidelity or Scottrade for trading penny stocks.
I've had an Etrade account for years w/o problems.
Make sure you research additional fees for trading penny stocks before you choose a new broker.
Keep in mind that most penny stocks and penny stock accounts can't be transferred from one broker to another. So, once you buy them, you're stuck with that broker until you sell them.