This is an example Python plugin for QGIS Server.
Python plugins support for QGIS Server has been introduced recently with QGIS 2.8 and it is enabled by default on most distributions.
For more informations on how to use Python plugins for QGIS Server, please refer to a series of posts on our company website:
http://www.itopen.it/category/gis/qgis/qgis-server/
Sample filters:
Exception: http://localhost:8000/?SERVICE=EXCEPTION
Watermark: any WMS GetMap
HelloFilter: prints messages to the logs
GetFeatureInfoCSS: injects CSS in GetFeatureInfo HTML response
ParamsFilter: prints messages to the logs