Trees and Shrubs
Cornell University woody plants database
http://woodyplants.cals.cornell.edu/home
Oregon State University woody plants identification system
http://oregonstate.edu/dept/ldplants/plant_ident/
North Carolina State University plant identification online resources: trees
https://plants.ces.ncsu.edu/plants/category/trees/
North Carolina State University plant identification online resources: shrubs
https://plants.ces.ncsu.edu/plants/category/shrubs/
Native Plant Trust (formerly New England Wildflower Society), Go Botany
https://gobotany.nativeplanttrust.org/simple/woody-plants/
The Noble Foundation plant image gallery
https://nobleapps.noble.org/plantimagegallery/woodies.aspx
Perennials and Annuals
UMass Extension garden plant identification
http://extension.umass.edu/plant-identification/
Native Plant Trust (formerly New England Wildflower Society), Go Botany
https://gobotany.nativeplanttrust.org/simple/non-monocots/
North Carolina State University plant identification online resources: perennials
https://plants.ces.ncsu.edu/plants/category/perennials/
North Carolina State University plant identification online resources: annuals
https://plants.ces.ncsu.edu/plants/category/annuals/
The Noble Foundation plant image gallery
https://nobleapps.noble.org/plantimagegallery/forbs.aspx
Grasses
North Carolina State University plant identification online resources: ornamental grass
https://plants.ces.ncsu.edu/plants/category/ornamental-grass/
Native Plant Trust (formerly New England Wildflower Society), Go Botany
https://gobotany.nativeplanttrust.org/simple/graminoids/
The Noble foundation plant image gallery
https://nobleapps.noble.org/plantimagegallery/grasses.aspx
Weeds
UMass Extension weed herbarium
http://extension.umass.edu/landscape/weed-herbarium
New Jersey weed gallery
https://njaes.rutgers.edu/weeds/
Penn State University Center for Turf Science plant identification
http://plantscience.psu.edu/research/centers/turf/extension/plant-id
Virginia Tech weed identification guide
http://oak.ppws.vt.edu/~flessner/weedguide/genusindex.htm
Weed Society of America weed identification
http://wssa.net/wssa/weed/weed-identification/
Common weeds of North Central States
http://fieldcrop.msu.edu/uploads/documents/Ncr607.pdf
All types
University of Connecticut plant database
http://www.hort.uconn.edu/plants/search.php
USDA Natural Resources Conservation Service plants database
North Carolina State University plant identification online resources
Longwood Gardens plant finder
https://plantexplorer.longwoodgardens.org/ecmweb/FindPlant.html
Chicago Botanical Garden plant finder
https://www.chicagobotanic.org/plantcollections#plantfinder
National Gardening Association plant finder
Missouri Botanical Garden plant finder
http://www.missouribotanicalgarden.org/plantfinder/plantfindersearch.aspx
Native Plant Trust (formerly New England Wildflower Society), Go Botany
https://gobotany.nativeplanttrust.org/full/
The Noble foundation plant image gallery
https://nobleapps.noble.org/plantimagegallery/woodies.aspx
HGTV gardens plant finder