I have used two online trading platforms, Sharebuilder and Scottrade.
Sharebuilder: I started with this one in College as the price was good at $4 a trade ($15.95 to sell) however your trades are only executed on Tuesdays. I believe Market Trades are $15.95. Online support is decent; they will get back to you in 24 to 48 hours.
Pros: Cheap if you are putting money in the stock market regularly using the dollar cost average model and don't need in depth research reports or tools. A little easier to set up than Scottrade from what I remember
Cons: Expensive to buy (Market/Limit orders) and sell (Compared to Scottrade), research reports are not available and you have to pay a monthly fee to see your gains/losses and some of the bells/whistles other sites give you free.
Scottrade: Started trading with them this year and am very happy with what I get for the price. Trades are only $7 and are executed immediately with market trades. Customer service is available online or at an office near you. Excellent research tools, you can download S&P or Reuters reports for most companies that are traded on there.
Pros: $7 dollar trades, gain/loss tracker, excellent research tools available. Invest in Mutual funds (some you have to pay a fee to invest in some you don't). Great Customer service at their offices and online, easy to use website.
Cons: Haven't found any.
You may want to check out
Zecco 10 free trades a month ($4.50 after that) if you have $2,500 in your account. Has anyone used Zecco?
