There are actually a number of options available. If you mean automated trading, the list of choices narrows, but there are still dozens of alternatives. If you include only open source automated trading systems (ATS), you'll be left with only a couple. I manage one of the FOSS ATS, JBookTrader (http://code.google.com/p/jbooktrader/). Feel free to join, we need more developers.
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There are actually a number of options available. If you mean automated trading, the list of choices narrows, but there are still dozens of alternatives. If you include only open source automated trading systems (ATS), you'll be left with only a couple. I manage one of the FOSS ATS, JBookTrader (http://code.google.com/p/jbooktrader/). Feel free to join, we need more developers.