MAA - Mathematical Association of America
This website is for math teachers and will give ideas for teaching lessons.
NCTM - National Council of Teachers of Mathematics
This website is for math teacher. It provides access to many articles on math education.
Desmos
This website is for teachers and students. Students can use this as a graphing and calculating tool. Teachers can use this as a graphing and calculating tool, as well as a teaching tool that allows you to create slides with problems. As the studets work on the slides, the teacher can view what each student is doing.