BackyardGardener.com: Garden Guides on Plants

Starting point for information on (types of) plants, with summary information for many species and varieties.
http://www.backyardgardener.com/plants/

WeedAlert.com

Offers a weed listing for the United States regions, primarily weeds occurring in lawns.
http://www.weedalert.com/

Findmeplants.com

Garden plant finder. Search by specifiying garden conditions and design requirements such as flower color, size and growth habit. Search results include plant photo and summary information.
http://www.findmeplants.com/

Cal's Plant of the Week

Features a new plant every week, with photo, description, and cultural information. Previously featured plants can be selected by botanical or common name. Focus is on plants for greenhouse culture.
http://www.plantoftheweek.org/

The Garden of Paghat the Ratgirl

Pacific Northwest gardener presents essays for hundreds of species growing in her garden, with personal notes, history, cultural information, and photos for each.
http://www.paghat.com/

FloraFinder.com

Offers links to gardening websites arranged by plant type, with a brief description of each site.
http://www.florafinder.com/

DenverPlants.com

This resource for gardeners in the Denver metro area includes a sizeable database of plants with photos and cultural information.
http://www.denverplants.com/

Plants Database

Online USDA database of botanical information, images, and links on plants in the USA, including crops and invasives. Search by name, geographic distribution, or other characteristics.
http://plants.usda.gov/

HortiPlex

Horticultural database with over 100,000 species and cultivars. Comments, ratings, and photos for many varieties supplied by members of the Gardenweb community.
http://hortiplex.gardenweb.com/

Plantfacts.com

Searchable database of plants with information on flowering, and climate.
http://www.plantfacts.com/

Gardenline

Guide to growing and caring for houseplants, vegetables, trees and shrubs. Also features common pests and a miscellaneous section. From the University of Saskatchewan Extension.
http://www.gardenline.usask.ca/

Echo: Fruiting Trees, Shrubs, and Herbaceous Plants

Features cultural information, uses, handling tips, and links for an extensive list of fruits and vegetables.
http://www.echonet.org/eln&herbs/eln_catalog/eln_index.htm

Fruits and Vegetables

Extensive set of fact sheets and links from the University of Florida.
http://edis.ifas.ufl.edu/TOPIC_Fruits_and_Vegetables

Horticopia On-line Plant Information

Extensive directory of plants, categorized by genus, includes summary cultural information and small photograph for each plant.
http://www.horticopia.com/hortpix/

Shiraz Gardeners' Guide

Gardening resource site with an extensive plant directory featuring photos and cultural information.
http://shiraz.me.uk/

Floridata

Features hundreds of plant portraits, each illustrated with several photos, documenting the garden uses and cultural requirements of each species or cultivar.
http://www.floridata.com/

PlantZAfrica

Extensive information about plants native to southern Africa, including hundreds of alphabetically arranged plant portraits, by the National Botanical Institute of South Africa.
http://www.plantzafrica.com/

Foliage Plants Database

Searchable database with basic information and some photographs of plants grown primarily for their foliage. Japanese and English.
http://albinopri.cool.ne.jp/leaf/en/

Gardenweb: Name That Plant

Forum that allows users to upload images of plants that they are trying to identify.
http://forums2.gardenweb.com/forums/namegal/

Dave's Garden: PlantFiles

Database with over 80,000 plants with cultivation information and pictures supplied by member gardeners.
http://davesgarden.com/pf/

Illinois' Best Plants

Chicago Botanic Garden provides a database of recommended plants for Illinois gardens, from perennials to trees, with photos and information on their culture and use.
http://bestplants.chicago-botanic.org/

All America Selections Winners

Showcases new varieties of flowers, vegetables, and plants for garden performance.
http://www.all-americaselections.org/

Plants For A Future

Resource centre for rare and unusual plants, particularly those which have edible, medicinal or other uses. Offers a plant database, and various articles on gardening, perennials, and habitats.
http://www.pfaf.org/

New Jersey Weed Gallery

Database of weeds, accessible by common and botanical names and by a thumbnail gallery. Summary information and photos are provided for each weed.
http://www.rce.rutgers.edu/weeds/

Plants Toxic to Animals

Resources about toxic plants from the Veterinary Medicine Library.
http://www.library.uiuc.edu/vex/vetdocs/toxic.htm

Translat

Translation program for botanical latin.
http://members.ozemail.com.au/~pbostock/

Desert-Tropicals.com

Features searchable database of thousands of plants, with basic information and often photos. Includes many succulents, herbs, and tropical fruit trees.
http://www.desert-tropicals.com/

Shady Character

Mark shares photos and descriptions with personal experiences growing a large number of shade plants, both perennials and woody specimens.
http://www.mailbag.com/users/mnofsing/shad.html

Virtual Plant Tags

Database of detailed descriptions and growing tips for thousands of plants, including many not commonly grown. Profiles include cultural information, assessment of landscape value, and photos for most.
http://www.virtualplanttags.com/

Tiscali Gardening

A South African flower, plant and gardening portal with searchable plant database, photographs and information.
http://new.mweb.co.za/gardening/

PlantBuzz

Gardener Mark McDonough presents horticultural musings and illustrated plant studies, with particular emphasis on alliums and other bulbs for rock gardens.
http://www.plantbuzz.com/