Image Zoomer With HTML5 Canvas: Wadda
It comes in 2 JS files, one for the common functions an the other for the wadda function.
The library simply displays the original-sized-image inside a "lens effect" that must be mentioned in the thumbnail's title tag.
Wadda has several options for customization like the zoom level, lens size, enable/disable lens fade effect and the x/y offsets from the mouse cursor.
It is compatible with every canvas-enabled browser and will work on others using the explorercanvas library.