Below are links specifically for teachers.
Prodigy: Free for teachers, common core aligned adaptive math site in a gaming environment. Information is reported back to you in real time.
Math Coaches Corner- A blog site managed by a former classroom teacher, math coach and interventionist. She puts up all things related to math instruction; great articles, talking points for discussion, books, strategies, links...you name it!
The National Council for Teachers of Mathematics
A valuable website of resources for teachers.