Well, I wrote a web app for this myself for a MUD I play. http://draconic.ca/RetroMap/ But that's probably a bit overkill. I was in large part using it as an excuse to play around with some things like OpenID.

If you don't mind managing the list of players manually, you just need to make a GeoJSON, GML, KML, or even a CSV file, and you can feed it into pretty much any of the major webmapping APIs. I favour OpenLayers myself, but Leaflet might be easier. You could also use one of the commercial ones like Google Maps or Bing.

Otherwise you'll need to write a web application to generate the file, and then display it with one of the Javascript APIs. That's what RetroMAP does, though you could probably build a much simpler one than I did and it would probably work about as well.