One of my students, Sam Curren, has put together a geospatial mash-up called ActiveTrails. I like it a lot--it's got everything you'd expect in a mash-up of this sort.
ActiveTrails shows hiking trails in certain geographic regions. The trails aren't just shown as trailheads or lines on a map--they're real GPS tracking data that people submit for their favorite trails. Right now, you have to send them to Sam and he uploads them, but he promises that users will be able to self-provision trails "real soon now."
Each trail has a place to leave comments, shows elevation data, and traces the trail on either GoogleMap or GoogleEarth. You can also see topo maps from the USGS. Very cool.
Here's a trail that my daughter and I hiked part of in June. We just went up to Big Spring and camped and then went back to the trailhead in South Fork Canyon.