MyGardenGuide

Offers plant database with photos and summary information, Q&A on various garden topics, and a garden blog.
http://www.mygardenguide.com/

The New York Botanical Garden: Continuing Education

Offers classes about plants, gardening, commercial horticulture and landscape design.
http://www.nybg.org/edu/conted/

Mimi.hu: Gardening

Provides a directory of biological and horticultural terms, with links to online encyclopedia entries for each term.
http://en.mimi.hu/gardening/index_gardening.html

Hort.net

Offers a collection of garden reference materials, including a plant gallery and profiles, archives of mailing lists, gardening news, and links.
http://www.hort.net/

Pioneer Thinking: Gardening Center

Information and tips on outdoor and indoor gardening. Many articles grouped by topic.
http://www.pioneerthinking.com/gardening.html

Biodynamic Gardening

Information on this method of gardening using mineral preparations and planetary rhythms, and services including workshops and garden design. Based in the UK.
http://www.peagreenboat.co.uk/

Yardener.com

Lawn and garden resources for homeowners whose garden is not their hobby. Includes information and care data for many plants, shrubs, and trees, and dealing with pests and problems.
http://www.yardener.com/

Exploratorium: Science Of Gardening

Features articles and videos on several topics, including gardening with nature, carnivorous plants, soils, fruits, and vegetables, propagation, and plant reproduction.
http://www.exploratorium.com/gardening/

Let's Go Gardening

General gardening site, with versions geared towards US and UK gardeners. Tips and articles, informational resources, links to events, societies, and public gardens, and a forum.
http://www.letsgogardening.co.uk/