PBS LearningMedia (was Teacher’s Domain) is one of my go-to sites when I’m planning a lesson. It puts all of the PBS content at your fingertips. You can search by topic and grade and find videos, interactive lessons, lesson plans and more. Although I still hang out on the Nova and Nature websites often, this is the best place to find just what you’re looking for. You can register for a free account and even register your students for interactive lessons.