For Modernizr, the site is here:
Other popular libraries are:
DojoToolkit ( http://dojotoolkit.org
Good if you want to target mobile apps, but has a lot more features than that. It has a widget framework that allows you to code whole web browser app. You can add things like calendars, date pickers, menus, toolbars, file uploaders and tons more.
MooTools ( http://mootools.net/
YUI ( http://yuilibrary.com/
Yahoo User Interface is a library on par with Dojo in that it's more widget oriented. Good community of users.