After a couple of months of frantic uploading and tagging, I have completely documented the Pacific Crest Trail (http://en.wikipedia.org/wiki/Pacific_Crest_Trail) from Mexico to Canada. That's 2650 miles of trail through 25 national forests, 7 national parks and 3 states. Some of the trail was already marked but most of that was wrong, showing only where the national Forestry Service had marked the trail on their paper maps and not where the trail was actually built. My data was all recorded by GPS when I thru-hiked the trail in 2007.
I've made it into one relation (http://www.openstreetmap.org/browse/relation/1225378/) and that seems too big for the server as the little preview map in the corner no longer shows the route, but Lonvia's Hiking Map does (http://osm.lonvia.de/world_hiking.html?zoom=5&lat=40.70331&lon=-98.88906&layers=FFBT) though of this writing it still hasn't fetched the last few hundred miles to the Canadian border. There are relations for each of the sections as defined in the PCT data book.
This was my first OSM project, and since I have no other major data source to play with it may be my last for a while.