"The My Maps feature of Google Maps lets people create maps to share their hobbies and expertise with the world. For example, a surfing enthusiast could map out their favorite surf spots or a surfing club could plot all the best beaches in Southern California. Now imagine if all the surfers around the globe worked together, leveraging their combined knowledge to create a single map of the best surf spots worldwide, applying the power of wiki-style collaboration to cartography," suggests Google Lat-Long blog.
And if you don't feel like creating a new map from scratches, you can now import it from a KML or GeoRSS file. To find one, search in Google Maps and select the "community maps" option at the bottom of the standard search results.
Then you can invite collaborators and send them a message. The dialog is very similar to the one from Google Docs, except that you can transform a map into a wiki. This would've been very useful for the San Diego Fire Map "that tracked the spread of the fire and included information on evacuation alerts and evacuation center locations", but it's also an interesting way to share your knowledge with other people.
As an example of collaborative map, here's a map where you can add a placemark for your location and maybe something about you. Just click on "Edit", find your location on the map, right-click and choose "Add a placemark".