The Transit & Trails site shows trailheads, campgrounds, and hiking trips that are accessible by transit in the SF Bay Area, and, recently, in the Santa Cruz area. Transit connections to locations are shown on 511.org and Google Transit through links.
Recently, the Bay Area Open Space Council, which has sponsored the development of the site, announced an iPhone app.
The Transit & Trails is interested in seeing the site implemented in other areas as well. The backend code of the site is open source.