eBay Inc. says it has made the programming code for the PayPal Inc. payment system available to external software developers.

Extending access to the PayPal code will inspire other companies to build the tools needed to make the payment service available on more Web sites, as well as on mobile phones and, eventually, in traditional retail stores, eBay's chief technology officer, Mark Carges, said last week at a conference in San Francisco.

eBay expects revenue from PayPal to double, to $5 billion, by 2011, as it steps up competition with MasterCard Inc. and Visa Inc. in online payments.