I'm looking for a deep discount broker. What are the most popular ones
out there? I'm just trying to avoid ending up with a fly-by-night
company that'll cheap me out of my money, and I figured going with the
popular ones was a good way of accomplishing it. Also, if you know of
any deep discount brokers that're especially good or bad, I'd
appreciate your comments.

They're not the deepest discount broker, but TD Waterhouse only
charges me $12 per trade (I'm fairly active).  I've been with them for
nearly 3 years, and their customer service has been flawless so far. 
When I had a problem making a trade on one occasion (I think now it
was my new software), they handled it on the phone and charged only
the $12 online rate . . . and they were pleasant about it.