If your community has any plugins installed, you can view a list of them on the Installed Plugins page. If you don't have any yet, Jive Software offers a few you can get started with. The Add Plugin page lists plugins that are available free from Jive Software. It also provides a place to add plugins for which you have a JAR file.
Admin Console: System > Plugins > Installed Plugins
Admin Console: System > Plugins > Add Plugin
Whenever you add or remove a plugin, you'll need to restart the application server.