By Region
About.com: Top 100 Trees, Ariadne's Trees and Forests, British Trees, Catalog of the Woody Plants of Oklahoma, Common Trees of Pennsylvania, Conifers in the Pacific Northwest, Forest Trees of Florida, Goo's Tree Encyclopedia, Identification of Common Trees of Iowa, Identifying Trees of Michigan
Tree Identification
About.com - A Tree Leaf Key, Bartosz Pacula's Pictures of Trees, Basics of Tree ID, Dendrology at Virginia Tech, Fact Sheets for 680 Trees, Foliage Identification Test, Introduction to Tree Identification, Palm Tree Photo Gallery, Plant Kaleidoscope, SelecTree for California
Forest Biology and Dendrology at Virginia Tech
Information about courses, trees, and forest biology. Includes fact sheets on over 450 species of trees.
International Dendrological Research Institute
Nonprofit organization dedicated to documenting the woody flora worldwide.
The American Woods
Wood sections of 350 North American trees from the 14-volume book The American Woods, published between 1888 and 1910 by Romeyn Beck Hough.
Tree Matters
Site for The Dendrologist, a quarterly magazine. Also contains tree tips and a record of the dates of upcoming dendrology events in the UK.
Tree Root Architecture
Information on research of tree root architecture, root excavation, 3D-digitising of root systems, tree root growth, tree stability, and windthrow.
TreeGuide
Includes information about almost all North American trees, with brief descriptions, range maps, and a few images. Brief family summaries for trees from elsewhere. List of state and provincial trees.