Who do you use? What do you like or dislike about the company? I tried out Scottrade, but didn't like it at all. Its basically the Wal-Mart of online brokers, no frills. TD Ameritrade and E-Trade seems to be pretty good. I would go with Fidelity, but to get the low commission rate you need $25,000 in your account, $5K shy. I am going to be day trading, so I need a trading platform that is fast and easy to use.
I LOVED Vanguard when I had an account with them. Although I wasn't a "day trader", I did make a lot of changes, and it was incredibly easy and straight-forward. I currently have an account with T. Rowe and Nationwide, and I like both (although I like T. Rowe much better). T. Rowe has a lot of options, and Nationwide is relatively simple.
I don't know about day trading with T. Rowe, but my individual account was easy to set up, and they didn't require an arm and a leg for me to open it. I have traded with it, and from what I remember (it's been a while--I leave it alone for the most part now), it was very easy. My account with Nationwide is through my employer, so I don't do much with it.
Edit: Oh, I also had a TD Ameritrade account, and it was okay. If you can get into Vanguard, I highly recommend them. As an individual, I believe they require quite a bit of money up front. It's been a while since I've looked into brokers... maybe I should try to switch back to them.