This is yet another Linux success story. PayPal says Linux grid can replace IBM mainframes:

PayPal is currently processing $1,571 worth of transactions per second ( that is $2,262,240 per day) in 17 different currencies on about 4,000 servers running Red Hat Linux. Thompson supervises a payment system that operates on about 4,000 servers running Red Hat Linux in the same manner that eBay and Google conduct their business on top of a grid of Linux servers. “I have been pleasantly surprised at how much we’ve been able to do with this approach. It operates like a mainframe,” he said.