General Plant Databases

Desert Tropicals
A copyrighted database of desert plants native to the USA. Be sure to reword any information garnered from here to make it an original creative work and thus avoid copyright issues.
PlantDatabase.co.uk
A copyrighted general plant database, mostly UK climate focussed. Note that although this is a user generated database, it is copyright protected. Be sure to reword any information gathered.
Center for New Crops & Plant Products
A database of less common species investigated for their value as agricultural crops. Indexed by species name. There's also a list of famine food species available.

Uses

Medicinal

Purdue University "A Guide to Medicinal and Aromatic Plants"
A great database of medicinal plants with some original sources archived to read online.
Annies Remedy
A commercial website selling herbal remedies, but with some pages of medicinal uses for common medicinal herbs and plants.
Dr. Duke's Phytochemical and Ethnobotanical Databases
Chemicals in plants and ethnobotanical uses of plants
A Modern Herbal
hyper-text version, first published in 1931, by Mrs. M. Grieve

Ecological Functions

Dynamic/Mineral accumulators and Nitrogen Fixers

OrgegonBD "Dynamic Accumulators of Nutrients for Composting"
A great table of mineral accumulating plants indicating which plants draw which minerals

Interactions and Polycultures

Companion planting lists

OregonBD "Companion Plants"
A list of companion plants - fairly standard information which can be found in many places. No information on the WHY's and all unreferenced.

Propagation

UC Davies Plant Sciences Rooting Database
A database of cutting rooting with environmental conditions, success rate, etc