Khan Academy: Thousands of videos providing coverage on a variety of content areas, most of which are math, science, and finance. Art and history included. Great way to reinforce skills, differentiate instruction, or review. https://www.khanacademy.org/
http://mathforum.org/
Wolfram/Alpha: A computation engine that provides knowledge and computes answers.
http://www.wolframalpha.com/
Khan Academy: Thousands of videos providing coverage on a variety of content areas, most of which are math, science, and finance. Art and history included. Great way to reinforce skills, differentiate instruction, or review.
https://www.khanacademy.org/
Utah State University: National Library of Virtual Manipulatives
http://nlvm.usu.edu/en/nav/vlibrary.html