Pssst.Hey kid,comere... Very wise to know who your enemy's are. A long story short.Knight Capital Group aka NITE is the Biggest MM of them all.They do what their name implies.Make Markets(put up they're own money to "make a market") for the sale of securities,mutual funds,options,hedge funds and forex.
Also,a little known fact is that the CEO of Scottrade is on the BOD's of Knight...Mmmm...