Marshall Kirkpatrick compares four geomapping services, quoting ClustrMaps (featured here before), Frappr (whom I have found a bit too US centric to be interesting to me -- if you don't have a US zip code, it has at least been a great hassle tagging their maps in the past), GVisit and Geo-Loc.
I mailed GVisit a (rather detailed) suggestion on how to make a JSON feed of their data, and quite promptly received an answer asking if I would want to write the code myself. The code was almost already there anyway, and I got a peek of the RSS feed generator to base it on, and it was live within the day. Which was nice. :-)
Stay tuned for more info on what you can do in a web page with a JSON geofeed.