The Xara Picture Editor is a completely modular application. Each of the tools is a separate part of the program. New modules or plug-ins can be provided by Xara, or by any third party developer wishing to extend the program. New tools appear as new icons on the toolbar. Adding new tools is as easy as copying a file into the Picture Editor folder.
Xara provide a detailed API and documentation for third party developers of add-on tools. See here.
In addition the Xara Picture Editor works with many Photoshop compatible plug-ins. There are thousands of free and commercial plug-ins available on the Internet.
Click the PS plug-ins link to the left for more information