"FNA": Flora of North America Editorial Committee, e. 1993-2011. Flora of North America. Oxford University Press, New York.
The Flora of North America is perhaps the reference for taxonomy and identification of just about anything growing north of the Mexican border.
"Tropicos": Tropicos.org. Missouri Botanical Garden. 2012 <http://www.tropicos.org>
Tropicos is an enormous database of taxonomy and herbarium-specimen data maintained by the Missouri Botanical Garden. I use it primarily for scientific-name data.
"IPNI": The International Plant Names Index (2008). Published on the Internet <http://www.ipni.org> [accessed January 2012]
IPNI, like Tropicos, is another huge taxonomic database. It lacks Tropicos' added specimen data, but it collects name information from most of the definitive sources in botany.
"CIC": Herbarium specimen data from the Harold M. Tucker Herbarium (CIC) at the College of Idaho. Data and images will be published through the Consortium of Pacific Northwest Herbaria, <www.pnwherbaria.org>.
"SRP": Herbarium specimen data from the Snake River Plains Herbarium (SRP) at Boise State University. Data and images will be published through the Consortium of Pacific Northwest Herbaria, <www.pnwherbaria.org>.