xfce4-panel - Add New Items
The Add New Items window allows you easily add new plugins onto your panel. It can be accessed by right-clicking on the desired panel and selecting Panel → Add New Items….
|Add New Items window|
Adding Panel Plugins
To add a plugin to a panel, select a plugin from the list and then click Add. The plugin you have added will appear at the end of the selected panel. You can use the search box to search for your desired plugin in the list.
Tip: You can quickly add an item to a panel by dragging it directly from the list and on to the panel you wish to add it to. If you want to place it in between two existing launchers, hold Ctrl.
Tip: To access the generic menu of the plugin (including access to the preferences) when it is not accessible by right click (such as for the taskbar), hold Ctrl + right click.
Apart from the plugins that come bundled with the panel there are quite a few more you may be able to easily install (depending on your distribution).
Browse the list of panel plugins