The National Council of Teachers of English's (NCTE) ReadWriteThink website provides educators with access to high-quality lessons in reading and language arts instruction by offering the very best in free materials.
For science, math, engineering, and technology, teachers can't go wrong with the University of Colorado, Boulder's PhET, which creates very accessible free interactive math and science simulations. These simulations are based on extensive research, and they engage students through an intuitive, game-like environment so they can have fun learning through exploration and discovery. Another very comprehensive site, PhET lessons are very engaging and easy to integrate into your classroom activities.
PBS LearningMedia provides interactive lessons in science, social studies, math, and English language arts. Lessons are updated frequently to reflect the real world, current events, and student interest. A recently shared math lesson, for example, helps students learn about the competitive sport of fencing, along with the resulting mathematical problems and equivalent expressions that help determine the number of matches played per tournament.