There are a lot of things to consider when choosing what brokerages to use. I use 2 - Scottrade and Interactive Brokers. Here are some of my reasons.
Interactive Brokers caters more to serious traders. You must deposit at least $10,000 to open an account. They have some really nice features, though. For example, if you want to create your own custom trading software, they have an API that you can use to interface your software to their server. They will also set up a paper trading account for you for free. They also have a lot of data you can download, some free, some not. Some of the free data you can download is 6 months of historical stock, option, and futures prices and volumes at up to 1 second intervals. They have a lot of advanced data that can be streamed like market depth. They also have decent commission rates. For option contracts, they have the lowest prices I have seen vs all of the other retail brokers I have looked at. For stocks, they charge $0.05/share, with a $1 minimum, which is really inexpensive until you start buying thousands of a single share at once.
Scottrade would be a good brokerage to use if you don't have much capital, since they will let you open an account with as little as $500. They don't have the bells and whistles though, like being able to download historical price data, and they don't have an API. In comparison to the basic retail brokerages, their commissions are better than most but nothing to get excited about, unless you want to submit a large stock order. They charge $7/stock trade for stocks over $1, and there is not a limit on the size of the order, so you could buy, for example, 15,000 shares of BAC and only pay $7 in commissions. I don't know any other broker that can get close to that. If you want to try Scottrade, they also give out free trades sometimes. If you want to try them out use the code 'AINA4998' when you set up your account and they will give you 3 free trades.
Personally, I trade stocks and options. Whenever I use margin, I will always trade with Interactive Brokers because their rates are a lot lower. Likewise, they will always get my options business because they are a lot cheaper. However, whenever I buy more than about 2000 shares at a time, I will be using Scottrade. I hope this helps someone.