Some additional resources and directories we find useful:

United States

USDA PLANTS Database - The PLANTS Database provides standardized information about the vascular plants, mosses, liverworts, hornworts, and lichens of the U.S. and its territories.

Ladybird Johnson Wildflower Center Native Plant Database - This useful database allows you to filter results based upon a number of parameters such as bloom time, light requirements, soil preference and more to find just the right native plant for your needs.

North American Native Plant Societies - a comprehensive list of native plant societies by state or province. Native plant societies can be helpful in finding local nurseries specializing in native plants and for providing suggestions and information about the plants that are native where you live.

Canada

David Suzuki Foundation

Canadian Wildlife Federation

Britain

Royal Horticultural Society

British Beekeepers Association

BBC

BugLife

Switzerland (and France)

Pronatura, Basel, Switzerland. A leaflet about creating a bee-friendly garden (in French).

Mexico

University of Rochester, Minckley Lab

Singapore

Plants in Tropical Cities, by Bee et al. This guide includes photos of 2,800 tropical plants (mostly ornamentals), with a list of those attractive to butterflies and to birds at the back.

Australia

Native Bees on Wildflowers in Western Australia, by Terry F. Houston (June 2000). Perth, WA: Western Australian Museum. p. 41. ISBN 0-646-39816-4

New Zealand

Trees for Bees. Plant lists for gardens and farms.

Kenya (East Africa)

Our Friends The Pollinators: A Handbook of Pollinator Diversity and Conservation in East Africa