Yahoo plans Tuesday to roll out a new set of APIs to allow third-party developers to build applications using Yahoo services.

The four new APIs provide ways to access data inside Yahoo Photos, Calendar, MyWeb, and Shopping.

Developers will be able to use the APIs to add features to these services, or to use Yahoo data in desktop applications or online mash-up applications. Yahoo is also opening an application gallery to showcase developers' work.

Yahoos APIs are currently available free of charge for noncommercial use. They have built-in bandwidth limits that Ash Patel, Yahoo chief product officer, characterizes as "very generous." Patel says Yahoo has been working with developers of particularly popular applications who want to commercialize their Yahoo-dependent apps on a case-by-case basis.

Patel notes that Yahoo has been working on a comprehensive plan for the commercialization of its APIs. The plan is intended to clarify what constitutes a commercial use and what doesn't. The plan will accompany a set of tools developers can use to monetize Yahoo's APIs.

"You'll see an announcement from us about this in the future," Patel says. "Our aim is that by opening up our platform, we'll really open up the pent-up innovation that exists in developers out there. We're hoping that people will actually be able to start businesses on top of our platform."

Yahoo plans to announce the APIs at the O'Reilly Emerging Technology Conference in San Diego.