What platforms do web based stock trading systems use in the background. eg www.wallstreetsurvior.com?

I want to create an interface that will use and already in place stock trading platform. I have looked at .net stock trader if you google it but it is only a demo and never made it to market. I would like something similar to that or if you know of something open source that would do the same thing for me I would really appreciate it. I have looked all over the web so any help would be greatly appreciated. Basically I want a trading platform that I can put my own user interface on and go from there whether its open source or I have to pay for it is fine. Any suggestions would be helpful (nb: I am not sure if Marketcetera will do this for me but any suggestions are welcom).

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