CactiGuide - A cactus database including photos
Calphotos Plants - Database of California plant photos
Carnivorous Plant Photo Finder - A collection of links to carnivorous plant photos
Cryptanthinae Systematics - Photos and keys for the Cryptanthinae tribe of Boraginaceae
Delaware Wildflowers - Photos of Delaware wildflowers
Flora of North America - Online version of the Flora of North America
Kansas Wildflowers and Grasses - Photos and information on Kansas plants
Henriette's Herbal Homepage - Herbal information site
Jepson Online Interchange - Online version of California's Jepson Manual of Higher Plants
Mark Egger's Flickr Collection - Photos by Mark Egger, Castilleja (paintbrush) expert
Minnesota Wildflowers - Photos and information on Minnesota plants
Missouri Plants - Photos and information on Missouri plants
Montana Plant Life - Photos and information on Montana plants
Noble Plant Image Gallery - Photos and information on Oklahoma and Texas plants
Oregon Flora Project - Devoted to creating a new flora of Oregon, includes interactive plant atlas and photos
Southeastern Flora - Photos of southeastern US plants
Southern California Wildflowers - Photos and information on southern California wildflowers
Southwest Colorado Wildflowers - Photos and information on Four Corners area plants
Texas A&M Vascular Plant Image Library - Photos of Texas plants
Thomas Schoepke Plant Image Gallery - Plant photos from throughout the world
USDA Plants Database - Database of United States plants including distributional data
WisFlora - Database of Wisconsin vascular plants

Other Biology Links:

Bug Guide - Identification, information, and images of North American invertebrates
Calphotos - They have photos of things other than plants too
Integrated Taxonomic Information System - Database of taxonomy