Hypatia Create: An amazing Google Add-on that makes adding high-end math equations easy and fast!
Mathigon: A free tool of courses, manipulatives to learn difficult math concepts.
Polyup.com: Polyup is a computational thinking playground. Students from elementary through high school can play with puzzles involving numbers and functions. In a drag-and-drop, expression-building environment, students will create expressions that evaluate to a target value or values. Polyup has recently integrated with Google Classroom. Teachers can more easily assign Polyup’s games, called Poly Machines, to their classes, and track progress as they complete these exercises. Check out Alice Keeler's page for more information on how to utilize this tool.