nrich.maths.org I love some of the resources here – good for finding games and puzzles that are math-centric rather than entertainment-centric.
teacher.desmos.com Well crafted activities for exploring math concepts. Mostly high school math. Parents can go to teacher.desmos.com; if there are a few learners they can all log onto the same activity and many activities are designed for interaction between learners. Learners go to student.desmos.com and type in the classroom code for the activity you have picked out. Check out the polygraph activities to get started (minimum two participants).
mathisfun.com I find this to be a good resource for finding explanations to any school math. Easy to navigate around.
Khan Academy, of course…
homeschoolmath.net A nice place to find printable worksheets for grades 1 – 9.