CK-12 https://www.ck12.org/student/

Khan Academy https://www.khanacademy.org/

PHET https://phet.colorado.edu/en/simulations/filter?subjects=math&levels=high-school&type=html,prototype

Desmos https://www.desmos.com/

Brilliant https://brilliant.org/s/math/

Skew the Script https://skewthescript.org/