Minecraft Map Viewer, HTML5 (no libraries), October 2012 onwards.
The application takes a large image, renders it in chunks at different zooms and shows annotations. The image in the deployed example is from a third-party renderer for Minecraft saves.
Compatibility note: During development it was tested in contemporary version of Chrome and Firefox. It still seems OK in current ones. It was never intended to work in IE, but modern IE should be much better than the one at the time, so you might get on OK.