I just came across this interesting site via Twitter:
It describes itself as a cross between Wikipedia and Google Maps for walking tours -- where you can create walking tours with Google Maps and then other people can edit and update the maps.
Here's a sample 14km walking tour of Prague:
You can also embed the maps in other websites with HTML iframes.
It looks like a useful site with a lot of potential.
There are also similar sites for bike maps like Bikely.com, Bikemap.net, and MapMyRide.com (though the bike map sites don't have collaborative editing).