One thought that excited me recently: the geographical information that is often saved in newer digital photographs via GPS, combined with photo tags on photosharing pages like flickr or just the context of photos, will lead to an implicit tagging of maps.
Probably the information will not be accurate enough to generate maps like "normal" maps from them (that show specific buildings and streets), but surely it will be possible to identify landmarks and other things that are not commonly marked on maps.

For example, I was never sure where or what exactly the "Cote D'Azure" is, so I searched for it on flickr maps, and it already gives me a rough idea of the area. Flickr maps was a bit unsatisfying, though. I suspect they show only a few photos, not all matches. Through the flickr forums I found another service, loc.alize.us that seems to rely on flickr data, but shows a few more hits. I am still not sure what exactly it shows. Anyway, for now there are usually just a few hundred hits, but soon it will be millions, which should provide pretty accurate information.

Another thing I searched for is "Kungsleden", the name of a trail in Sweden that I walked once upon a time. So far, there are not that many pictures, but you can already see where the path probably is, and I am sure soon it will be a very precise map. It also seems to work quite well for the Camino de Santiago (Jakobsweg in German), at least for the Spanish part.

Another idea to play around with would be the spreading of plants animals, or of fashion and architectural styles. And this article on the effect of names reminded me of a project a former colleague told me about, where he extracted addresses from the phone book to aid his father's research in the etymology of names. Apparently a lot of interesting things can be derived from names, like how much people moved around in former times. With geotags, it might soon be rather easy to get that information (Facebooks photo service might be very interesting for this, because it encourages tagging pictures with the name of the people who are in it).

I know this thought is probably not very cutting edge. GeoTags have been around for a while, and people are already building services around it (like loc.alize.us). Still, I am intrigued, also because some nice mathematical problems might be involved (for example, determine the real location of something if pictures have been taken from different places. Discover what different tags mean the same thing). The current maps don't really satisfy my curiosity, and they are probably geared towards different purposes.

I wonder how much data Flickr is giving away through it's API? I already checked, and the API is not ideal: when you query for a photo, the response doesn't seem to include the information that if is GeoTagged or not, and it is also not possible as a search parameter. So one has to make an extra check for every photo and get as a response the geotag if it exists, or the info that it doesn't exist. Still, I didn't find anything about traffic limits. I don't know what other photo sharing sites are out there that could be interesting. Also, I wonder if online newspapers and blogs commonly include the geo information in the photos they display. (Another search idea: "accident" could reveal hotspots for accidents).

I'd love to play around with this. So many things to do... But we'll see, perhaps I'll actually get around to it eventually. In any case I am now more sure than ever that for my next camera (or camera enabled phone), I want to have that GPS tagging feature. Recommendations are welcome.