Kottke has a link to a Desire Paths Flickr pool this morning.
According to Wikipedians, the desire path/desire line is:
"A path developed by erosion caused by animal or human footfall. The path usually represents the shortest or most easily navigated route between an origin and destination. The width and amount of erosion of the line represents the amount of demand."
I love this. It makes me happy to think that strangers are forging useful paths together.(I guess that can be said for Wikipedia as well)