If you buy and sell stocks online, you want the quickest possible trades at the best possible price. Brokers promise you the sweetest deal available, but how can they guarantee that?

Their trick, in more and more cases, is to route the trade through Gerald Putnam's Archipelago. The Chicago firm, which created one of a new class of trading systems known as electronic communications networks (ECNs), can instantly determine where the best price is—on traditional markets, like the New York Stock Exchange, AMEX and NASDAQ, or among orders that come directly into Archipelago. Trades on the fully automated system are completed in less than a second, while a typical trade on the N.Y.S.E. takes 22 sec. "That's a lifetime in an active market," says CEO Putnam. "Our advantage comes from having no human intervention." Investors have been able to trade NASDAQ stocks via Archipelago since 1997, and the system got a huge boost this summer when it became the first ECN to quote stocks listed on the N.Y.S.E. and AMEX.