Discovery Education is a great resource for teachers. There are free, and fee-based resources available.
Free:
Discovery Educator Network (DEN) - the DEN is a community of educators who use Discovery Education resources in their classrooms. Members share lesson plans, tips, ideas, and more. I recommend that all teachers join. It's free. You can even become a STAR Discovery Educator.
Discovery School - in this section, you can find lesson plans, curriculum ideas, puzzles, worksheets, a guide to internet resources, lesson planner, science fair tips, and the New Teacher Survival Central, among other resources.
There is also a section with homework help available.
Discovery Partnered with CDW-G to create a whole section on Web 2.0 resources for educators and how to use them in your classroom. I highly recommend this section.
There are also resources for professional development.
Fee Based:
Discovery Streaming (once know as United Streaming) - schools or districts can subscribe to Discovery Streaming and gain access to tens of thousands of videos, video clips, graphics and lesson resources. Videos can be streamed over the internet, or downloaded for later viewing. Videos come from Discovery's excellent series as well as many other publishers.
Discovery Science - a great resource for elementary and middle school science teachers and students
Discovery Health - health topics and resources
Discovery Education has a huge amount of resources. I've only covered some of the resources. Head to the links and explore it for yourself. You'll be amazed at the resources available to you.
From my experience, all of them are worthwhile and relevant and very helpful.