Hello,
Google offers an open API for displaying custom information on their highly usable map application. This API seems suitable for wikis, which is why there are already three MediaWiki extensions for Google Maps. But these extensions seemed a bit too rigid for me, needing database modifications and RSS feeds and the like just to display markers. So I wrote a simple Google Maps extension: to create a map with many labeled markers, you just describe one marker per line with the format "latitude,longitude,label", and put all of these markers inside of a <googlemap> tag. A neat feature is that the marker labels will render wiki formatting. Because of its flexibility, I think this extension will prove useful in a wide variety of projects; I encourage you to try it out and send me feedback.
http://meta.wikimedia.org/wiki/User:Emiller/GoogleMapsExtension
Evan Miller