Today, UMapper are announcing that they are supporting custom maps. Any static map image can be uploaded to UMapper to be converted into an interactive map. The new functionality is already active, and the official announcement is due in the next few hours.
Here is a video explaining the process:
The use of static maps in online map applications is already possible using OpenLayers, and with Bing Maps using MapCruncher. MapCruncher also adds sophisticated quad tree tiling, but continues to be officially research software.
Interestingly, the new UMapper feature can work with non-map images, and UMapper even include demonstrations of it being used like this. A "map-like" control would be useful for non-map images in applications such as panoramic photographs, astro-photography, and other wide area high-resolution applications. It is unclear if quad-tree tiling is supported for deep-zoom performance.