Free AMC 8 & Math Kangaroo mock tests with solved past papers, hints & full solutions. Middle-school math olympiad prep — pick a contest to begin.
American Mathematics Competitions, middle-school level (grades 6–8).
Math Kangaroo (Canguru), a worldwide contest spanning grades 1–12.
A little red dot of math — made by a middle-schooler, free for everyone, out to punch above its weight. One account works across both.