Millions of people around the world use Yahoo! every day. One reason is that a single user id and password gives them access to more content and services than you can get anywhere else. In fact, we small companies often say things like "if we only had a fraction of your users..."
Now they can. Anyone can.
Our Browser Based Authentication (BBAuth) is a generic mechanism that will allow users to grant 3rd party web-based applications access their Yahoo! data. There's already a similar mechanism in place on Flickr and used by services like MOO. BBAuth is the protocol that's going to open the door to doing the same thing for many Yahoo! branded services in the coming months. Stay tuned for those announcements. :-)
Beyond that, BBAuth also makes it possible to use Yahoo! as a single sign-on for your site, thus removing a barrier to entry for a whole lot of people (over 200 million to be exact). This is still fairly experimental, so we'd love to get your feedback and input on how to make it even more useful.
See Also: Launching the Un-Launch-Able (Dan's blog)