http://www.seadragon.com/ service and its quite impressive in terms of sharing high resolution images online.
Below is our 10000x5000 pixel render of the 3D London Tube Map - it enables you to zoom in enough to see the parts we need to fix :)
If you have Silverlight installed it should zoom in nice and smoothly, if not it should use Ajax so we would be interested in any thoughts on its performance.
Set up is as easy as pointing the service at a online image, it then processes it and makes it as simple as adding in a one line script to a blog post / web page. We are not sure however know where it stores the images so if its a photograph you don't want sitting on a Microsoft server somewhere you might want to use other alternatives - such as our Image Cutter :)