Google Doctype is an open encyclopedia and reference library. Written by web developers, for web developers. It includes articles on web security, JavaScript DOM manipulation, CSS tips and tricks, and more. The reference section includes a growing library of test cases for checking cross-browser and cross-platform compatibility.
The Aptana IDE is a free, open-source, cross-platform, JavaScript-focused development environment for building Ajax applications. It features code assist on JavaScript, HTML, CSS, FTP/SFTP support and a JavaScript debugger.