GIS Cloud HTML5 Canvas
This Google Map demonstrates the advantages of using HTML5 canvas to create an overlay for a map.
What impresses me most about the map is how fast the canvas element is drawn on the map. There are a few thousand polygons on this map and they load seamlessly. If you zoom in and out on the map you can really appreciate the speed with which the polygons are displayed.
I think the overlay shows US monthly employment rates.