Plant Illustrations -- at plantillustrations.org
Max Antheunisse in the Netherlands has hand-crafted a collection of copyright-free botanical illustrations from various centuries, under a Creative Commons license.
The collection is not yet comprehensive, but is large and easily explored. The site provides a searchable index so that we can find illustrations by searching on botanical name, publication, artist, etc.
I especially like the index list of artists, linked to their works.