Sometimes, you just gotta build your own...
http://plants.usda.gov/
This is probably the largest, but that's a guess, public plant database, searchable by common, and botanical names, from species to plant kingdoms, if anyone knows of other good reources, I guess you might want to post it here. It includes extensive search criteria.
heres a couple others
http://davesgarden.com/pf/
http://www.rain-tree.com/plants.htm