This script uses the Aster GDEM elevation data set from GeoNames to work out the height of a geocache in feet above sea level and adds it alongside the cache's coordinates on its Geocaching.com listing page.
I wrote it so that I had a relatively easy way of figuring out which geocache I had found was the highest, without having to pull out the atlas. It's handy if you want to know how high up a mountain you need to climb to find a cache. Unfortunately, it won't help at all if you are trying to figure out how far above or below ground level the cache is actually hidden!
(Screenshot from metres version).
It also works below sea level!
If you prefer heights in metres, there is another version of the script that does that.
If you are new to userscripts, there is a HOWTO with instructions on how to get started.