I randomly happen to stumble upon this very cool and interactive website (by the folks at the Bay Area Open Space Council) which allows you to trip plan using public transit to Bay Area hikes and campsites.
The site is still in beta but has a good amount of trips detailed.
You start by using the Find tab, to search for hikes by an area, from a list of "featured" trips -- or by trailheads. For campers, you can also search by nearby campgrounds. Visitors to the San Francisco Bay Area will appreciate the search function, which will allow you to search by zip code or city name with a mileage radius (ie. Search 94114 within 20 miles).
After selecting a hike, you can see the detail of the hike or use the "Go on this Trip" function which then links to 511.org or Google Transit to show how to get to/from the hike using public transit. Using 511.org seems to work a bit better.
They are still working on the ability to share hikes - but there are plenty of good hikes already listed on the site. (Remember, its still in Beta - there was only one glitch I saw: if you enter Colma it will take you to a map of Colmar, France)
In any case, its a great tool to find how to get to the hikes around the Bay Area without having to drive your car. Check it out here, Transit and Trails.org