HTML5 provides the geolocation api. This is commonly used to move the map to the position of the user. Quite a useful feature, but obviously it is not limited to this. I made a quick HTML5 site which will give the height of you location according to the AHN - the dutch height model.
It is very simple: the geolocation API will provide a coordinate and I use that coordinate to do a getfeatureinfo on the AHN-service of EduGIS. To make it slightly more interesting it will show a cow if you’re above sea level, a nice coral reef if below and a foggy picture if the app doesn’t know where you are, or doesn’t have a height for that location.
Obviously this only works for the Netherlands, since the AHN only provides heights for the Netherlands.
You can find your own height here: http://research.geodan.nl/sites/hoogte/