In some financial markets there are Clearing Houses. These act as central counterparties, and thus reduce or eliminate settlement risk, and often allow netting of transactions.
Please submit sites of clearing houses. Stock exchanges should be listed at http://dmoz.org/Business/Investing/Stocks_and_Bonds/Exchanges/ and futures exchanges at http://dmoz.org/Business/Investing/Commodities,_Futures/Exchanges/ .