XP Server is an app server based on XPCOM and Apache. Requests are made to the Apache server and handled by a module. The module reads XML template files and then uses XPCOM to create a SAX chain of components to handle the request. Components can be written in C, Java, or Javascript. There are existing components for XML, XSLT, 3D charting, GD graphics, FOP to PDF and database integration. XP Server was originally written for a dotcom venture that didn't make it, the code has been donated to mozdev.org and it's looking for new owners.
http://xpserver.mozdev.org
