Experienced programmers may also want to take a look at this  StackOverflow thread  on the subject, but most of the resources there aren't free.

Eloquent JavaScript

Eloquent JavaScript  is a JavaScript book by Marijn Haverbeke. The digital form is free, and a print version is forthcoming. The book assumes no prior programming experience. Of the books listed here, this one might have the most to offer experienced programmers looking to get started with JavaScript.

Sams Teach Yourself JavaScript in 24 Hours

Sams Teach Yourself JavaScript in 24 Hours  by Michael Moncur is part of the well known series of Sams  books.

Learn to Program with Javascript

Learn to Program with Javascript  is a series of tutorials from  About.com . As the title implies, no programming experience is required.

W3Schools JavaScript Tutorial

W3Schools  is one of the most venerable and respected resources for online tutorials, and of course it has its  own JavaScript tutorial . It also has a stash of  JavaScript examples , including some advanced scripts.

Wikibooks JavaScript

The  Wikibooks  book on  JavaScript  isn't finished yet, but it's one to watch. If you're an experienced JavaScript programmer looking for a way to give back to the community, maybe you could contribute to this book.

SitePoint's JavaScript Tutorials

SitePoint is another well respected online source for tutorials and books. It has a few free tutorials, mostly as previews for its books. Its  JavaScript Guru List  is a good place to start.

Bonus: Douglas Crockford's JavaScript Lectures

Well known JavaScript expert and  JavaScript: The Good Parts  author Douglas Crockford has a series of free lectures available  on history of JavaScript, its features, and its use.