Plant ID

Las Pilitas Nursery specializes in natives. They provide very detailed (and entertaining) info on all the CA natives and a great resource guide for birds and butterfly plants.
Dave's Garden is a great all-around plant resource guide. Look up just about anything.

Companion Planting

A highly detailed chart on vegetables, fruits, herbs, and flowers that work together or against each other on Wikepedia here.
The 3 Sisters Planting Method explained here.

Vermicomposting (composting with worms)

Everything you ever wanted to learn about composting with worms from Cornell University.
Other critters you'll find in your worm bin are examined here.
Take a class on composting or get more information locally at the Solana Center for Environmental Innovation.

Nutrition from the garden

Plug in a food at nutritiondata.self, and find some really cool charts to compare nutritional value. Find out a food's "Amino Acid Score", "Completeness Score", and more.

Gardening Videos

Watch A Growing Passion, a KPBS program, produced locally.

Resource for classes or gardening help

The San Diego Master Gardeners host class seminars and have a master gardener waiting to help you.

Local Gardens with fun stuff for kids

San Diego Botanical Gardens (formally known as Quail Gardens) has everything from waterfalls to fantastic climbing trees.
The Water Conservation Garden in El Cajon -- beautiful, educational, and fun for the whole family. Plus, they just opened a Butterfly Pavilion.