University of West Florida's Resource and Information Guide is devoted entirely to lesson planning:
UWF Libguides -- Lesson Planning: General
ARTSEDGE: The National Arts and Education Network
Cyberbee
Discovery Education
EDSITEment (National Endowment for the Humanities)
Education World
Eduref.org
Illuminations (National Council of Teachers of Mathematics)
Library of Congress - Teachers
McREL Mid-continent Research for Education and Learning
National Archives
National Geographic Education
National Science Digital Library
PBS Teachers
ReadWriteThink
Science NetLinks
Smithsonian Institute for Educators
Teachable Moment
Teacher Vision
Using Primary Sources - Library of Congress