OpenLayers is already able to plot geo-referenced images as a layer.
Zoomify (http://www.zoomify.com/default.htm) is a free application that takes an image and supplies a folder containing a collection of tiled versions of the image at a range of resolutions.
A layer capable of plotting multi-resolution tiled data will offer high performance when viewing a backdrop image at a range of zoom levels. Zoomify EZ is able to produce this data at zero cost.
It may be possible to duplicate this functionality using a WMS and TileCache, but this implementation is more accessible to the non-WMS user.
This feature would be directly applicable to plotting scanned maps/charts.