I've had a stock investment account at Sharebuilder for several years now. I first opened my account at Sharebuilder because I liked their prices, and I still like their prices. For the basic package, there's no monthly fee and no minimum balance, and I can buy stocks on Tuesdays for just $4 per trade. I can also make real-time trades for $10 per trade, which is useful if I think the share price is going to move quickly if I don't jump on it.
I don't trade no-load mutual funds at Sharebuilder because they charge $20 per trade for that, and I can invest in most mutual funds directly for no extra charge. Recently, Sharebuilder has started offering option trades as a part of their basic package at $10 per trade + $1.50 per contract, which is a competitive price with the other online brokerage firms. You can also set up a margin account now at Sharebuilder, but I don't buy my stocks on margin, so I've never set up that feature.
One of the features that I really like about Sharebuilderis that I can set up the $4 Tuesday trade for the stock that I want to buy, and if I don't have enough money in my money market balance yet, Sharebuilder will wait until I have enough money saved up in my account before it executes that Tuesday trade. I also make a lot of $10 real-time Limit orders when I'm interested in buying a stock, but I refuse to pay more than a certain price for it. Sharebuilder will also let me set up Stop-Loss orders to sell a stock automatically if the share price drops too much.
I don't recommend Sharebuilder for stock research or advice. It's passable, but I can find more detailed information about stocks and companies from Yahoo Finance. But I do recommend Sharebuilder for small accounts that are just starting out and need a discount online brokerage for trades.