What is it?
Illuminations is the teacher resource website for the National Council of Teachers of Mathematics. It has lesson plans and interactives.
Who is it for?
This website is made for teachers of math. The interactives are meant to be used by students, but are not really set up for students to navigate and find appropriate interactives on their own.
Why use it?
Each of the over 600 lessons is clearly tied to a standard, so it is clear what the purpose of each lesson is. They also publish a newsletter and have games, applets and virtual manipulatives.
The search function of this website is great. You can search by grade, or by math strand in case there was something your students were struggling with and you are not sure what grade level that skill/topic is usually taught. They also have some really challenging brain teasers.