Explore Outdoors is a free layer for the Android and iPhone augmented reality app "Layar." It displays the names of mountains, lakes, parks, streams and a whole bunch of other good stuff. Explore Outdoors is helpful for those times when you're sitting on top of a mountain and you wonder about the names of other nearby mountains. There's an example screenshot of it to the right, but it's a pretty terrible shot right now. I'll get a better one soon (if you've got one, send it to me and I'll use it).
The data currently only covers the US, and comes from the USGS Geographical Names Database which is published by the authority which official names these things. The database is served to users from this website, which is in no way affiliated with the US government. I also took all the icons and images used in the layer from public domain sources (or sources which I hope are public domain). My list of image sources is at the bottom of this page.
Some features I'm considering adding are the ability to search for locations by name, and the names of non-US geographical features. If you're interested in the latter and you know where I might find a database of those names, let me know.