It used to be 9.95 per trade, then 8, and now 4.95. Commissions can add up and suck the profitability out of your stock portfolio, especially if you want to make multiple trades per year or had in mind to trade in smaller batches of shares.
Robinhood has leveled the playing field with $0 commissions on trades. You can check out Robinhood at https://robinhood.com from your smartphone to sign up. You can also ask someone who already has Robinhood to give you a referral link and you both will receive a free share (usually of a cheap stock).
I personally like using Robinhood the most for option trades, where commissions really eat into my profitability especially if the options only cost under $100. The tools to research are missing from Robinhood so you will have to find some other tool to do your stock research before trading. Regarding the tools I like Fidelity and Charles Schwab better.