Gateway to 21st Century Skills - Originally begun by the National Library of Education, this is an excellent source of lesson plans and teacher resources at all levels. Once you've run a search or browsed a category, use the facets on the right of the page to limit to particular subjects, materials, grade levels, etc.
Higher Education
Tools for Teaching and Learning - The Schreyer Institute for Teaching Excellence at Pennsylvania State University offers a variety of tools for teaching and learning in higher education.
PBS Teachers - Can browse through the teacher resources by broad subject area.
Teachers Network - An alliance of education professionals based in New York provides resources for educators such as lesson plans. The "How To" area is probably the most helpful.
Study Stack - Tools to help individuals memorize information in various subject areas.
CyberBee - Suggestions on how to integrate web resources into teaching a variety of subjects.