Here on demo.qgis.org you will find some examples of QGIS-Server related projects. We have some examples of small example pages with tiles rendered by QGIS-Server, but also more sophisticated webclients getting there (WMS or tile) data from QGIS-Server!
This map of South Perth from Nathan Woodrow is based on a set of shapefiles, styled in QGIS Desktop and served with QGIS Server.View
This map of Swellendam (South Africa) from Tim Sutton is based on OpenStreetmap data, styled in QGIS Desktop and served with QGIS Server in a Leaflet client.View
This map of Alaska is showing the QGIS demo data styled in QGIS Desktop and served with QGIS Server in a Leaflet client.View
Example of a QGIS Web Client application connecting to a QGIS Server of the Canton of Solothurn in SwitzerlandView
A simple leaflet (leaflet from cdn) example, world data from QGIS serverView
An example of QGIS-Web Client: Kartenportal JenaView