MSN is letting outside developers and Web publishers connect to its services and applications via application programming interfaces (APIs) for MSN Search, Messenger and Virtual Earth.

MSN Search APIs will be open for non-commercial use; MSN Virtual Earth beta APIs will be available free for commercial use, with an option to later add contextual advertising; the MapPoint Web Service 4.0, which underlies Virtual Earth, will be available for a fee; and the MSN Messenger Activity API will let developers create applications that include IM.

While Microsoft continues to build out its client/server/software strategy, it will keep its eye on the Web-as-platform trend, in which Web services APIs allow developers and other companies to hook into a company's data and applications.

"There's been such overwhelming developer interest in MSN Virtual Earth, we'll open up to developers a new control for commercial use," said Steven Lawler, head of Microsoft's MapPoint business. The new control will be based on asynchronous JavaScript and XML (AJAX), a Web development strategy that allows the browser to make calls to a Web server to update information without having to reload the page.

Microsoft is playing catch-up here: in June, both Google and Yahoo announced APIs for their mapping services, allowing developers and publishers to use maps and the underlying geospatial technology. For example, housingmaps.com combines information from Google Maps with craigslist's classifieds to show rentals, home sales and sublets in a graphical interface that makes it easy to understand where a property is located.

And web companies like eBay and Amazon.com have found new revenue opportunities and new markets by offering Web services APIs.