If you are looking for real-time bid/ask/size quotations (as opposed to just price), check my post under the subject title "Level 2".
To cut to the chase:
Ameritrade, $2000 minimum to open, $10/mo for Level 2
- note, if you collect Continental airlines frequent flyer miles, you can pick up points for enrolling $2.5k, $5k, or $10k
Microcapfeed, $100/mo
Quotestream, $55/mo
- I just subscribed to this and used it for a full day. It works. I'm not a fan of Java applets, which they use; but so far no big complaints. (I can't seem to copy & paste from applet window to clipboard.)