The Internet is a great resource, but dog-eared, folded corners, flagged, and book marked, these remain a wonderful way to start planning.

101 Great Hikes of the San Francisco Bay Area, Ann Marie Brown
A newer revision of this book is available.
Peninsula Trails, Jean Rusmore and Fances Spangle
South Bay Trails, Jean Rusmore and Fances Spangle

Web sites (primary)

San Francisco Bay Area Hiker
Trail descriptions and reviews. Curated by Jan Huber, each hike includes location of the trail head, summary of the hike, detailed dtscriptions, and photographs. Trail heads are shown as markers on a Google map and organized in several searchable lists. The site is well indexed by Google.
I only recently discovered this site and I'm still learning it's in and outs. The Trailfinder allows you to search by length, difficulty, and features. Searches can be constrained by distance from a location. The site is very polished.
This site lets you draw a track on a Google Map and provides information on the distance and elevation covered. Very useful while trying to figure out a new hike or a variation on one a documented hike.

This started as a simple mashup showing some of the cool functionality enabled by the Google Maps API. However, recent changes have cluttered the layout to make room for ads and the site can now be difficult to use on a small (laptop) display.

This is a relief-shaded topographic map of the US overlayed with geographic data from the OpenStreetMap project.

I prefer this site to the Google Maps Terrain layer for its shading, clearer contour lines, and higher zoom levels.

The Landscape layer at this site is my second favorite for on-line topographic maps. The geographic data is also from the OpenStreetMap project.

A particular advantage of the OpenCycleMap is that it covers the whole world and not just the US. Off setting that is that it has a lower elevation resolution than TopOSM. Note that the contour lines and elevation marks are in meters.

Web sites (less used)

These links are sites that I have used in the past, use intermittently, or simple note for future use.

I was really excited by this site when it first appeared. It is well integrated with an Android app and it is easy to document your hike with uploaded photos. However, when it changed to using Adobe Flash, I largely quit using it.

The site is still useful for finding hikes and the guides can be quite detailed and informative.

450 Hikes in Bay Area, California
This site purports to be A comprehensive guide to hikes for every season and my initial reaction is that it meets its goal. The hikes are organized into categories that make it easy to browse for an interesting hike. I haven't used this site much, probably because it does not produce a lot of hits to my Google search patterns.