Runtime Environments

AppJet, Inc.
Users type JavaScript in Web browser box; code can then execute. Programs reside on AppJet servers; source code is public; all start as clones of extant programs. Like Lively Kernel. Descriptions, demos, examples, weblog, forum.
http://appjet.com/

deployZone
Cross-platform e-mail, database and web server, supports Server-side JavaScript to build Internet applications in an object-oriented, web-based content management system.
http://www.deployzone.com/

DMDScript
Implementation of ECMA 262 scripting language; standalone and ActiveScript versions available. By Digital Mars.
http://www.digitalmars.com/dscript/

JSLibs
Standalone JavaScript development environment; goal: turn JavaScript into a general-purpose scripting language. Has set of native modules that contain many general-purpose classes and functions. Open source, GPL.
http://code.google.com/p/jslibs/

Rhino - JavaScript for Java
An open-source implementation of JavaScript written entirely in Java.
http://www.mozilla.org/rhino/

Sun Labs Lively Kernel
Web programming environment, by Sun's Project Flair. Goal: give web programming equal simplicity, generality, flexibility as used in desktop programming, but with no installation, upgrade issues. Like AppJet. FAQ, forums, interactive tutorial. Open Source, GPL.
http://research.sun.com/projects/lively/