Quote from nyc-hotshot:
Here are a few that are gone (as far as I know):
Blackwood trading - They were a Wall st prop firm doing 20 million + shares a day
Broadway trading - Another Wall st firm that used to have a ton of prop guys & volume
Tradescape - Bought out by Etrade
Protrader - Bought out by Instinet for a bunch of money and then folded shortly after (Instinet kept the software)
Castle Online - No idea, but they disappeared
Shamrock - They've been gone a long time
Execute direct - Small firm, but gone also
Rom-bo - They're gone
ETS Securities - Small firm
There were many more, just can't remember anymore.
My guesses for next to go:
Cybertrader - Not really going, it will just be know as Schwab
Sonic trading - I think they are going soon
Trade Portal - Used to be cutting edge, now just gonna be cut
AB Watley - Used to be a huge firm, now teetering on extinction