I came across the Blogmapping service the other day. This is a neat use of the Google Maps API, allowing you to search for blogs be geographical location. I like it better than GeoURL, simply because of the visual nature of the application. You can even add your own sites.
The Boston Subway Map is the nicest application of the API I've seen yet. It's not that it's fancy - it isn't. It just provides a great service, very well done, that loads quickly and is quite usable. The problem with some services is that they load very slowly, making them less useful. I don't know what everyone is doing in Boston, but Joe Hughes also has a great addition to your toolkit with a mashup for planning trips. The MBTA Nearby Stops Experiment will plot the nearest stations to the center of the map, and the pop-up info will even include upcoming trains at that location. Very cool.
Which brings us to the New York City Subway Map offered by onNYTurf.com. Very nicely done, but something about the page makes it load really slowly, unlike the Boston map above. There are other NYC subway maps out there - MonkeyHomes.com has a nice one - but this one is a better use of the API, with colored paths and the like.
Finally, if you want to add that special touch to each and every page you visit, take a look at the Greasmap script for Greasemonkey. It adds a little map to each page where it finds location information. Very nice.