I need to include any dll in my asp.net project?.Is that good to use in web page my page..
YUI is a javascript framework. You do not need any serverside stuff like a DLL for it. It is good for your web page if you like to work with it and if it does what you need.
Some alternatives would be jQuery+jQueryUI or ExtJS.
There are a few server-side options, there's a project on codeplex for a few wrapped controls as asp.net controls or the YUIAsp.Net library, if that's what you're after.
For general usage, you just need to include the library in the page and go from there, you can host it on your site or use a CDN:
As for others, I'd check out jQuery and possibly jQuery UI, there are others, but after a lot of testing, I settle on jQuery personally, it's clean/concise and tons of community support, that doesn't mean you should rule out others though, see what's best for you,