JPS: JavaScript Password System
A Javascript password script written by P.J.Wise & A.J.Chee. You will need to download it to use it, but you can view a demo on the site. Their readme page also has more information and instructions for setting up the script on your site.

Password Protection
A JavaScript password script written by S. Scott Brady. Uses an external javascript to do the work, and then display the page. The author states that the script is not totally secure, so do not use this to protect important information.