Tree ID
Arbor Day's 'What Tree Is That': https://www.arborday.org/trees/whattree/
Ohio Public Library's "What Tree is it": http://www.oplin.org/tree/index.html
LeafSnap Species: http://leafsnap.com/species/
Virginia Tech App: Virginia Tech Tree ID App

General Links
Arbor Day Foundation: http://www.arborday.org/
Alliance for Community Trees: http://actrees.org/ 
Asian Longhorned Beetle: http://asianlonghornedbeetle.com/
Continental Dialogue on Non-Native Forest Insects & Diseases:
Emerald Ash Borer: http://emeraldashborer.info/
International Society of Arboriculture: http://www.isa-arbor.com/
Plant a Billion Trees: http://www.plantabillion.org/
Society of Municipal Arborists: http://www.urban-forestry.com/ 
University of Georgia Center for Invasive Species & Ecosystem Health: http://www.bugwood.org
USFS Urban and Community Forestry: http://www.fs.fed.us/ucf/
Utility Arborist Association: http://www.utilityarborist.org/