Clicker, Inc. (fka CLKZ)

[gap434]

This hasn't been updated since August, but can still be used as a guide:

Clearing Firms By Broker
Here are the main clearing firms used by our reviewed brokers for US traded securities:

Broker Clearing Firm
E*TRADE: E*TRADE Clearing LLC
eOption: Penson Financial Services, Inc.
Fidelity: National Financial Services LLC (a subsidiary of Fidelity)
Firstrade: Penson Financial Services, Inc.
Interactive Brokers: self clearing
Just2Trade: Legent Clearing LLC
OptionsHouse: Penson Financial Services, Inc.
optionsXpress: self clearing in the US, Penson in Canada
Merrill Edge: Merrill Lynch
Schwab: Charles Schwab Clearing Services
Scottrade: self clearing
ShareBuilder: Pershing LLC
SogoTrade: Penson Financial Services, Inc.
TD Ameritrade: TD Ameritrade Clearing, Inc.
TradeKing: Penson Financial Services, Inc.
Vanguard: self clearing
WellsTrade: First Clearing, LLC (a subsidiary of Wells Fargo Advisors)
Zecco: Penson Financial Services, Inc.

http://www.onlinebrokerrev.com/2011/01/broker-clearing-firms.html