JSJaC is a jabber client library written in JavaScript to ease implementation of web based jabber clients.
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.
piwik is an open source (GPL license) web analytics software.
csscss will parse any CSS files you give it and let you know which rulesets have duplicated declarations.
A SASS mixin for writing media queries with a clean and natural syntax
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.