The move is part of Microsoft's efforts to drum up support for its Windows Live hosted services.
There is also a fee component to some of the services after a certain limit for use is reached. Burk acknowledged that Microsoft hopes to develop a broad partner community around Windows Live so eventually the company will earn revenue through this new API structure.
Microsoft began its ambitious rebranding and relaunch of many services formerly branded MSN to the Windows Live moniker in November 2005. The initiative is part of the company's Online Services Business, the revenues of which grew slightly in Microsoft's recently reported 2007 third quarter, from $562m (£281m) last year to $623m (£311m) this year.
Most observers agree Microsoft faces a tough battle to grow this business, which is being ramped up to drive online advertising revenues in competition with Google and Yahoo. Microsoft executives have asserted it is a long-term investment for them and they are willing to be patient to achieve growth.
The APIs do have an online advertising component, Burk said. API users can set up a revenue-sharing plan with Microsoft by offering advertising through web pages and services it builds on the company's APIs, she said.
Currently, the APIs for Windows Live services are in beta and available for free, although some have limits. Once they emerge from that state, they will still be available for free, but some will have related surcharges and limits, Burk said.
Virtual Earth is available for free for as many as 3 million map tiles a month, according to Microsoft. Silverlight Streaming, a new service launched this week for multimedia streaming and hosting, is free up to 4GB of storage and unlimited outbound streaming, with no limit on the number of users who can view streams.
For Windows Live Contacts, Windows Live Spaces and Windows Live ID, once the APIs are out of beta, Microsoft will charge 25 cents per user, per year for anything above 1 million unique users per month, Burk said.
Microsoft has 30 billion contacts and 500 million addresses stored for its web-based services, Burk said. Companies that leverage the Windows Live Contacts, Windows Live Spaces and Windows Live ID APIs will have access to these.
In addition to these APIs available now, Microsoft will release a web IM Presence API for integrating Windows Live IM and user web presence into sites in the next two months, Burk said. All the APIs should be in full release some time next year.