yahoo! has 60 million users, making it one of the top three IM companies. Together with the release of their Messenger 8.0 for Windows, Yahoo released a development kit to allow third parties to create content plugins for Yahoo IM. A good example is the release of Jeteye for Yahoo! Messenger, where Jeteye has become a plugin for the popular IM client. Created plugins can be viewed here : Gallery. Plugins can be submitted to the Yahoo! Developers Network where they will be published. I think this is a real Web2.0 development where companies are allowed to interact with Yahoo! and add functionality to software that they need. Until now, you could not do much more than stick to features that come with the software.
Link : Techcrunch