A nice demo of a javascript-based client is available at the National Gallery showing a couple of paintings in high resolution. This uses a test version of the next server release incorporating several new upcoming features such as contrast control and 16 bit image handling. The demos also allow the user to blend between colour and X-ray images.
The VIPS image processing system can generate the Tiled Pyramidal TIFF images required for IIPImage and is designed to efficiently process the kinds of extremely large images we can handle.