Linked to STEM and Common Core Standards – NSDL is the nation's online portal for education and research on learning in Science, Technology, Engineering, and
information from many sites is compiled on HippoCampus. It is a one stop for info Math, Natural Science & Social Science.
An interactive periodic table for students.
This site has all sorts of K-4 worksheets for Language Arts, Math, and Social Studies/Science. It even has Teacher Tools like printable lesson planners, calendars, logs, etc.
With a library of over 2,600 videos covering everything from arithmetic to physics, finance, and history and 210 practice exercises, they are on a mission to help you learn what you want, when you want, at your own pace.
The US Mint has several money/finance related lesson plans for grades K-12 in Social Studies, Language Arts, Science and Mathematics.