Jonathan Stott has been hard at work over at EarthTools.org writing a contour map overlay which looks pretty snazzy and compliments the height finder tool already on his site. His coverage is growing all the time and he aims to cover the whole world at zoom levels 10, 9, 8, 7 and 6 and then selected areas at closer zoom levels. To see the areas of the world that have contour maps, check here. To use these maps from his main site toggle the "contour" map control when you're exploring.
Jonathon explains some of the technical challenges in creating this additional mapping layer:
For example, I cover the whole of the western United States up to zoom level 13 as well as an area of the Alps in Europe. The United Kingdom and Ireland as well as Cyprus are covered up to zoom level 14. One limiting factor is the speed of my computer - it will take about a month to generate all the tiles for zoom level 10 for the whole world. The other is the space on my webserver - the maps currently take up about 20GB. Another slight problem is that I use an alpha transparency layer for the PNG images so that I can darken the satellite and still see the contours well enough. This doesn't work on Internet Explorer 6, so I have given people the option to install FireFox or people will have to wait for Internet Explorer 7 (the current beta shows the contour maps well).