General Information on Particular Plant Species:
Encyclopedia of Life: http://eol.org/info/plants
The link will take you to an introduction page on plants; you can use the "search eol" function at the top of the page to find information on particular species. This project was started in 2007 and is funded by a large number of organizations with an interest in biological diversity. Authoritative information, assembled by scientists with expertise on each species, extensive documentation of citations and sources. Some of the language is technical, assuming a grounding in basic botany.
i-Tree: http://www.itreetools.org/
A tool set developed by the USDA to help communities assess the benefits of their trees.
Detailed, well-referenced information on the habitat, life history, and genetics of 200 forest tree species, most of them native to North America. USDA Agriculture Handbook 654.
A tool to estimate the monetary benefit of street trees.
Wild Plant Conservation:
Plantlife International http://www.plantlife.org.uk/international
An organization involved in plant conservation efforts around the world. The parent organization, Plantlife, began in the United Kingdom and maintains a fascinating website (http://www.plantlife.org.uk) that is brim-full of information on British wild plants.
Pennypack Ecological Restoration Trust: http://www.pennypacktrust.org/
A a nonprofit conservancy in the Huntingdon Valley, northeast of Philadelphia.
Virginia Native Plants Society: http://vnps.org
A searchable site full of detailed articles on Virginia native plants.
Medicinal Plants
The professional organization of herbalists specializing in medicinal use of plants.
Kew Gardens Site on Medicinal Plants: http://www.kew.org/science-conservation/plants-fungi/medicine
Searchable by Latin and common species names. Images and detailed information on many medicinal plant species.
Plantlife International information on medicinal plant conservation:
http://www.plantlife.org.uk/international/wild_plants/medicinal_plants_and_livelihoods
Plants for a Future: http://www.pfaf.org/user/default.aspx
Lists medicinal uses and edibility of 7000 species. Sources for information are cited and there is an opportunity for users to comment on the information.
Other Websites:
A botanist and forager blogs about finding and using wild plants.
Weekly blog-posts on plants, written by self-declared "plant-nerd" Dan for what he calls "the phyto-curious".
An educational blog about plant biology, includes access through twitter, youtube, and tumblr. Several posts per month that combine intriguing photographs with snippets of botanical information.
A series of BBC podcasts by professor Kathy Willis, director of science at the Kew botanical gardens, covering the economic and ecological importance of plant diversity and genetics.
A blog highlighting tidbits of colorful information about plants and a series of 45 minute podcasts on topics in plant biology.
A series of short, entertaining videos on plant biology produced by Chris Martine at Bucknell University and co-sponsored by the Botanical Society of America.
Free e-books on botany