piwik is an open source (GPL license) web analytics software.
JSJaC is a jabber client library written in JavaScript to ease implementation of web based jabber clients.
FireBug is a new tool for Firefox that aids with debugging Javascript, DHTML, and Ajax. It is like a combination of the Javascript Console, DOM Inspector, and a command line Javascript interpreter.
AquaPath is a free Cocoa-based developer tool for Mac OS X Tiger that allows you evaluate XPath expressions against any XML document and view the result sequence in a dynamic, intuitive tree representation.