Labels
Community modules at this point are very informal. We've got this section of the wiki, where each should be documented, and we've got a module in the source code called 'community'.
New community modules
If you are interested in creating a community module, create a child of this page detailing what the module will do (if you have trouble creating a page because you don't have the right permissions you can skip this step for now, but should create a page after you get the rights). Then email the geoserver-devel list to ask for commit rights and describe your module. Your commit rights will be limited to the module you are working in, though could easily lead to full commit rights if your work is incorporated as part of the main GeoServer distribution. Though there is no requirements that community modules need to work to become part of the distribution, they can just be useful modules that others might want to use and/or collaborate on.
If your community module becomes mature please document it with instructions of how others can use it. We can also give you release rights on SourceForge, so you can make it available for download as a plug-in that others can include in their GeoServers.