This is the place for you if you need to identify a plant or want to find something unique to grow in your garden. Lots of photos, drawings and other information about plants, plants and more plants.
Harmful Plants (Rutgers)
This is a very low-key commercial website that focuses totally on perennials, to include an encyclopedia with complete descriptions of over 2000 perennials and photos.
Plant Data Base (U. of California - Berkeley)
Plant Data Base (USDA)
Plant Photo Pages (Texas A&M)