Center For Aquatic And Invasive Plants (University of Florida) - Search plants by common names, scientific names, and plant types.  Color photos with descriptions.

Maine Field Guide to Invasive Aquatic Plants (and their common native look alikes) - This is one of the better guides out there containing plant structure, color photos, descriptions, and look alikes.

Robert W. Freckmann Herbarium (UW-Stevens Point) - Color photos and descriptions of plants found in Wisconsin.

Virginia Tech Weed Identification Guide - Search plants by common name.  Contains plant descriptions, color photos, and identifying characteristics.