There are a lot of resources for educators and students around the web, but it can be very time consuming finding the ones you want. In this Techlearning.com blog post "Top 10 Sites for Educational Resources", David Kapuler lists and describes what he considers to be the best educational resources. His list has excellent resources for K-12 teachers, including: Thinkfinity, Teachers.net, Shmoop, Educational World, and more ...
For other teaching resources at various levels of education, you might also want to check out these lists of open educational resources (OER).