Math Forum at Drexel: Not a new site, but great resource for ideas, math talk, lesson plans, etc.
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