engine from TrimPath is a lightweight GPL open-source component that lets you have template-based programming (like PHP/ASP/JSP) while running in a web browser.
integrates with Firefox to put a wealth of web development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page.
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.
ajaxWM is a window manager in a web browser, a terminal emulator and an SSH proxy. It allows remote controlling a computer even when there is a firewall and packet analyzer between, blocking everything else than HTTP(S)-connections. Because the ajaxWM client runs in any javascript-capable web browser, you can even use it when you are not allowed to install any software, for example in an internet café, at school, or at work.