js Calender

This one is probably the best I've seen and has some cool features like multiple date selection, but the best feature it says on the site, that I liked is that it does not depend on any 3rd party library, I haven't tested it but it good on the site
http://www.dynarch.com/projects/calendar/

I downloaded the source, that has a really cool demo,
got to their downloads page http://www.dynarch.com/projects/calendar/download/
much more stuff at  http://www.dynarch.com/