CK-12: Math concepts for grades 1-5.
IXL: TEKS-aligned content covering grade-specific concepts from grades pre-K through 12, as well as Algebra I, Algebra II, geometry and precalculus.
Math Playground: Math games designed for grades 1-6.
Mathigon: Interactive textbooks, animations, and videos geared toward grades 6-12.
Mathway: Solves and explains math problems entered by the user. Includes solutions for algebra, calculus, trigonometry, and more.
PhET: Interactive simulations for teaching and learning math.
Texas Instruments (TI): Free six-month software subscriptions for students and teachers.