Cybraryman has a list of 70 Diigo social bookmark sites created by educators for educators. You could probably spend a lifetime here, but you'd learn a lot more than randomly search the internet or googling.
This website is great for everyday use in the classroom. This website covers everything from cultures, state capitals, current events, games. Kid friendly
Lots of worksheets and activities. Reading comprehension is broken down by grade level. Also has Spelling, Math, Language Arts, Science and Social Studies.