Official Documentation Available
This topic is now covered in Modules.
Magnolia's module concept makes it easy and powerful to build custom modules to extend Magnolia's functionality. To read more about it, head over to our official documentation site .
The Modules developed by Magnolia can be found here: http://svn.magnolia-cms.com/view/community/modules/.
You can make use of our infrastructure (jira, svn etc) if you wish to develop modules. See http://forge.magnolia-cms.com/.