General:

The U.S.D.A. PLANTS National Database.

Penn State's Directory for Invasive Weeds of the Northeast.

Weed Identification: a joint project of the University of Georgia's Center for Invasive Species and Ecosystem Health and the Weed Science Society of America.

University of Missouri: Weed ID Guide.

Purdue University: Common Weeds of No-Till Cropping Systems.

University of Maryland: Lawn Weed Identification.

NCSU Turf Files.

WeedAlert.com: Weed Identification.

Iowa State: Weed Science.

The Noble Foundation Plant Image Gallery.

Wildflowers:

Wildflowers of Northeastern & Northcentral U.S.A..

My Wildflowers: Wildflowers from the Trails of Western Pennsylvania and Beyond.

Wildflower Information .org.

The Wildflowers in bloom photo album.

Field and Swamp: Wildflowers.

Grasses:

Penn State Extension: Common Grasses Found in Pennsylvania Hay and Pasture Fields: Identification Matrix.

Aquatic Plants:

University of Florida Center for Aquatic and Invasive Plants.

Non-Flowering Plants:

Lichens of North America.

Fungi of Delaware.

Pennsylvania:

The Pennsylvania Flora Project of Morris Arboretum.

Wildflowers and Trees of Western Pennsylvania.

Pennsylvania: Common Garden Weeds.

New Jersey and Delaware:

The New Jersey Weed Gallery.

Delaware Wildflowers.

North:

Mound: Our Weeds (NYC).

The Connecticut Botanical Society.

Wildflowers of Rhode Island.

The UMass Weed Herbarium.

South:

The Flora of Southwest Virginia.

Wild Flowers and Plants of North Carolina.

West:

Prairie Wildflowers of Illinois.

Idaho's Noxious Weeds.

Iowa State University: Iowa's Noxious Weeds.

Further West:

Utah State University: Range Plants of Utah.

Far Away From Home:

Plants of Styria.