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/

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/

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/

Garden Terms

Gardening dictionary with brief definitions of a large number of terms, optionally arranged by category.
http://www.gardenterms.com/

MyGardenGuide

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

Garden Ideas

Articles and advice on a variety of gardening topics.
http://www.gardenideas.com/

LoveToKnow Garden

Short informational articles on gardening topics, vegetables, and ornamental plants, contributed and edited by users.
http://garden.lovetoknow.com