Online Book: A Modern Herbal 
From the web site, this is the online version of "A Modern Herbal, first published in 1931, by Mrs. M. Grieve, contains Medicinal, Culinary, Cosmetic and Economic Properties, Cultivation and Folk-Lore of Herbs."  This site provides information on more than 800 varieties of herbs and plants.  The original book was 860 pages. 

The Virtual Foliage Page 
From the web page, this is "your direct link to thousands of plant pictures."  The only catch is you have to know the scientific name of the plant you are looking for.  Then you can find it in the classification listed in the Taxa to Genus file. 

Here's an example.  To find pictures of American ginseng, you need to know that its scientific name is Panax quinquefolius, which you can find out from the Native American Ethnobotany Database.  Panax is the genus, quinquefolius is the species.  If you go to the Taxa to Genus file, you'll find that Panax is in Phylum Magnoliophyta, Class Magnoliopsida, Subclass Rosidae, Family Araliaceae.  Using those names you can click through the folders in the Plant Systematics Teaching Collection.  The final folder that contains the pictures is http://www.wisc.edu/botit/img/bot/401/Magnoliophyta/Magnoliopsida/Rosidae/Araliaceae/
Panax/Panax_quinquefolium/   (Note: the real folder name won't have a break or a space in it).
It takes a bit of time to figure out, but this is probably the best collection of plant photographs on the Internet.