can I use windows 8 font （Segoe UI）for my web app? [closed]

plus, how to know whether I can use any font for app development purpose? I mean develop app for commercial use, like develop app for client.

any free font website to recommend?

Segoe UI requires a license if you want to include it as a web font for commercial use in a site. Consider a font such as Open Sans as a free alternative or purchase the license. http://www.fontsquirrel.com/fonts/open-sans

There is now an open-source variant of Segoe UI, called Selawik. You can get it from here. It isn't available yet as a web font, but I think it's only a matter of time before it gets uploaded to a CDN.