These are lists of plants put together by various organizations. A few are in databases I manage; the rest are links to other websites.
Colorado Flora
Boulder County Flora
Flora of North America
The Plant List
NRCS Colorado Grasses
NRCS Colorado Fabaceae
NRCS U.S. Gymnosperms
NRCS Wetland Monocots
NRCS Ericaceae
The Colorado Native Plant Society
USDA Celebrating Wildflowers