I could go on about some of the cool moving parts in the offering - the Navigation Tab and Breadcrumbs design pattern look particularly useful. But the important takeaway from this isn't the actual features of the libraries, but the "design pattern" Yahoo! is adopting in making them available.

These libraries represent a lot of work. How does Yahoo! come out ahead by giving these tools away? There's the goodwill to be gained from folks like me who can and will put their tools to good use, for free. It's good karma. But also, and more importantly, it elevates the playing field for web apps in general. If Yahoo!'s UI tools and attendant design patterns proliferate -- and there's good reason to think they will -- the whole concept of "web app" will be upgraded. As the overall sophistication and power of web apps increases, who stands to benefit the most? Premium web services will be able to do things: 1) push the envelope for the apps and services they offer to their customers, but also 2) benefit from the availability of high quality web apps and services from partners and even competitors whom Yahoo! integrates with.

In this ecosystem, the general quality of the apps that are floating around matters. If you offer a *platform*, it's important that the developers who connect with and build mashups on top of your services have what they need to make your platform shine.

Even then, there's still something more. When you allow for both the good karma and the elevate-the-ecosystem arguments, the sense remains that Yahoo! developed this wicked cool library, and the developers just understood that it deserved much wider use than it would ever receive, even inside a huge web company like Yahoo! As developers know, sometimes a thing is just too good to be kept in a cage -- damn the economics, it deserves to be free.