• About.com: Botany - A page of links and articles.
• Agriculture Knowledgebase - Source for information on topics related to agriculture, biotechnology and allied areas including plant breeding, genetics, seed technology and tissue culture.
• Bioversity International - Provides information, publications and news on a range of subjects including agricultural ecosystems, genebanks, germplasm collection, crop wild relatives and neglected and underutilized species.
• Botanical Authors Database - Search the database of authors who contributed to the contents of the catalog of type specimens in the Harvard University Herbaria.
• Botanical Publications Database - Database of publications used to verify the entry of publication names in the Harvard Herbaria catalog.
• Botanical Society of America Image Collection - Collection of 14 sets of educational images for instructional use, with search capability.
• Botany Encyclopedia - Includes a botanical dictionary, advice for gardeners, and a store selling plants.
• Center for Plant Conservation - Collaborative organization dedicated to conserving and restoring the rare native plants of the United States. Includes information on conservation techniques.
• Glossary of Botanical Terms - Contains more than 2,500 terms relating to botany, gardening, horticulture and landscape architecture. Fully searchable and it is regularly updated.
• Hunt Institute for Botanical Documentation - A division of Carnegie Mellon University. Archives, research, art, and information about the institute are included.
• Plant Tissue Culture Information Exchange - Intended to give researchers, teachers and producers rapid access to information about plant tissue culture.
• Suite101: Botany - Articles on a wide range of plant topics for teachers, students, amateur naturalists and general readers.

Other links and resources here.