Yahoo's IM (instant messaging) network is being opened up to external developers so they can create more applications to improve its functionality.

Yahoo will today release a beta version of Yahoo Messenger with Voice, along with software development tools for creating plug-ins for it. The IM beta version will have some plug-ins already installed by default, and allow users to choose others.

Until now, Yahoo has provided APIs (application programming interfaces) that let developers work with its IM network to expose presence and buddy list information on a website, for example. However, the tools give developers broader access to the service, according to a company executive.

"This is the opening of Yahoo Messenger to developers," said Jeff Bonforte, Yahoo's senior director of real-time communication.

Rivals AOL and Microsoft also let third-party developers create applications for their respective IM networks. Internet companies such as Google, AOL, Yahoo and Microsoft have in recent years gone out of their way to entice external developers to create applications on top of their products and services. Developers have been particularly keen to build on Google's online mapping service, Google Maps.

By letting external developers do these 'mashups', internet companies are able to extend the functionality of their websites and networks in ways that may not have occurred to their own programmers. In the case of Yahoo Messenger, Yahoo wants users to add plug-ins to customise the service and increase their interaction and collaboration with other users, Bonforte said.

Among the plug-ins that the company has already developed is one that integrates the Yahoo Calendar with Yahoo Messenger, he said. From the Yahoo Messenger interface, this plug-in will let users view and manage their calendar, compare it with other users' calendars and query the company's local search and mapping service to find business listings and obtain driving directions.

Also available will be an Amazon plug-in, so that users can share gift wish lists and browse popular and new products. Users can check out plug-ins here. Yahoo expects to have hundreds of Yahoo Messenger plug-ins before the end of the year, Bonforte said.

The Yahoo Messenger developers tools are available to individual developers, as well as to developers working on behalf of a corporation, and will be placed here.

The new beta version of Yahoo Messenger, which can be downloaded here, will also feature other enhancements, including increasing the number of contacts to a maximum of 1,000 and the ability to add sound effects to voice conversations.