This is a good one for searching:
The PFAF Database
On our website, pfaf.org, you can search for over 7000 edible and medicinal plants using a number of search criteria including: common and Latin names, keyword, family, habitat and use (medicinal, edible or other).
Search techniques includes:
search by name
search by keyword
you can browse plants common and Latin names by alphabetical letter
you can browse plants by their family, habitat and use (medicinal, edible or other)
you can search a plant by it use, for example whether it can be used for:
edible: e.g. coffee, chocolate, gelatine, oil
medicinal e.g. acrid, antacid, antibiotic, kidney
other e.g. alcohol. beads, bottles, fencing, fuel
special uses e.g. nitrogen fixer, hedge
You can do a more detailed search using the Search Properties section. This allows you to search for a number of plant features at once. For example you might want to search for a plant that needs light sandy soil, that is between 1m and 5m high and that likes shade. The database will find plants that have all 3 of these features.