We have a repository at http://plugins.b2evolution.net/, where you can browse plugins by category and version.

For plugin developers, there is also a central source code repository (Subversion / SVN) hosted at SourceForge that you may use if you don’t want to maintain your out source code repository.

The source code can be browsed online at http://evocms-plugins.svn.sourceforge.net/.

Of course, you do not need to add your plugin here. You can also just submit your plugin to the standard plugins site.