I personally like ShareBuilder and have been with them since I turned 18. It has since merged with ING Direct. What I like about ShareBuilder is they have a lot of tools on their web site, and it's pretty easy to navigate. The trades are $4.00 if you sign up for automatic trades on Tuesdays. If you want one instantly, it's about $10 -- down from the $16 they used to charge before they merged.
With the merger, they now also have mutual funds as well. So it's nice having options. If you open an account with ING Direct, you could also add that account to your ShareBuilder account and invest in CD's should that be an interest for you.
I've also tried other brokerages and did a lot of research as well. Zecco I considered, but it didn't really appeal to me. Seemed to be too many "strings" attached. TD Ameritrade, E*TRADE, Charles Schwab, SoGoInvest et. al tend to have way too many strings attached.