CommunityWalk.com has really sprung to life with over 30 communities that have now been created. CommunityWalk gives you the ability to create your own Google Maps overlay. Uses range from local mountain biking locations to non-profit groups plotting locations of their members.
An example of this is how the Eastern Ontario Freecycling Group that has made extensive use of CommunityWalk's functionality by plotting the locations of their member groups. The info-balloons feature info about the group and link to the associated Yahoo! Group. Based on this being the top CommunityWalk "community" with over 8000 visits, it's clear that people are making use of this.
Meanwhile LiveJournal user Swamysk has used CommunityWalk to create a ">map of historic sites and places of interest in India.
It should be an interesting place to check out in a few months. I suspect many of the self created mashups that I'll be checking out and talking about here on Google Maps Mania will originate here.