Trade exchanges took a deep breath after the blockbuster announcement that Detroit's Big Three would merge their $300 billion-plus annual purchasing activities on a single Web site. The move raised the bar for an emerging class of "exchange" sites that aim to facilitate trillions of dollars' worth of manufacturing purchases.

"How will the big players make those cooperative sites work after years of competition?" asks Bob Parker, an e-commerce analyst at AMR Research Inc. His prediction: Independent trading exchanges will eventually dominate Web-based B2B services. Here are his top five, taken from a recent study of more than 600 such exchanges.

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