Releases is a project mainly written in PHP and JAVASCRIPT, it's free.
Xoops Engine code
/XoopsEngine The root directory for Xoops Engine code
/XoopsEngine/docs Documents
/XoopsEngine/htdocs Web docroot
/XoopsEngine/lib Libraries
/XoopsEngine/var System variable data
/XoopsEngine/usr User or community contributed components, including apps/modules, plugins, applets, themes, etc.
/XoopsEngine/usr/apps Xoops Engine apps, i.e. applications following new MVC
/XoopsEngine/user/modules Non MVC style modules
/XoopsEngine/usr/plugins Plugin folders
/XoopsEngine/usr/applets Applet folders
/XoopsEngine/usr/themes Theme folders