Note that this page is aimed primarily at K-12 teachers. Those teaching at higher levels will find appropriate resources at Useful reference & subject sites for college students & teachers.
Online Masters in Education has created a list of the 100 Best Web Resources for Educators in 2012. It’s divided into preschool, elementary, middle, and high school.
The Guardian website has a section called the Teacher Network that "gives teachers access to 70,000 pages of lesson plans and interactive teaching materials", including a bank of testing material that you can use to build your own tests and deliver online or in print.
For those interested in the Waldorf approach to learning, a new website inspired by Waldorf Education provides age-appropriate movement activities and a number of articles.
Worksheet Library is a nicely done site by a group of teachers, with a large collection of free and paid worksheets for grades K-6
Happy Child, who have a lot of good stuff on their own account, also provide a list of links to various good educational sites
the Gateway to Educational Materials: a directory to high quality lesson plans, curriculum units and other education resources on the Internet (and you can specify that you only want free stuff)
A to Z Teacher Stuff: a directory of online resources for teachers
PBS offer a directory of lesson plans
LessonPlans4Teachers claims to have created the best lesson plan directory and search site on the Internet
Education World offers lesson plans and professional advice to teachers (U.S.; elementary level)Study Stack: flash cards in various subjects
Teach-nology offers a lot of resources to subscribers, but also has a number of free worksheets, worksheet makers, graphic organizer makers, and rubric makers
The BBC site is broken down into age-groups: Preschool; 4-11; 11-16; 16+. UK students can also find several revision guides for different exams (including a Welsh one, and guides for Scottish exams)
Kid Info calls itself a reference resource for teachers, parents and students, and seems to have an impressive number of links, categorized on simple, clutter-free pages. [American]
Mindful Teachers has a number of resources to help teachers incorporate mindful activities into the classroom.
Vanderbilt University has a teaching guide for mindfulness in the classroom, mainly aimed at university educators, but probably useful for other teachers
Skepticism 101: The Skeptical Studies Curriculum Resource Center is a comprehensive, free repository of resources for teaching students how to think skeptically.
English Zone: grammar, spelling, vocabulary, advice on writing. Includes a section for teachers, with worksheets, lessons, quizzes, classroom games and activities, etc.
The National Gallery of Art offers Italian Renaissance Learning Resources for teachers
Art History Teaching Resources (AHTR) is a peer-populated platform for art history teachers with an extensive range of lesson plans and resources
New York's Museum of Modern Art, MoMA, has resources for teachers
the Metropolitan Museum of Art has lesson plans to help teachers integrate learning about works of art in the classroom