Mozilla

The Mozilla Foundation provides support for various software projects, notably:

Firefox, a powerful web browser
Thunderbird, a graphical email and news client
Bugzilla, a software bug tracker that runs as a web application
Sunbird, a calendar project
Mozilla Toolkit is based on the XML-based XUL user interface language which is used in conjunction with a layout engine called Gecko; this is what is used to implement much of the functionality of these applications as well as to implement extensions to them.