To me khanacademy is on of the best resources -- there're loads of videos and various tests and quizzes https://www.khanacademy.org/math
also MIT open course ware offers quite a number of interesting math related course
https://search.mit.edu/
also i like quizzes from https://www.theguardian.com/education proprofs.com yourhomeworkhelp.org softschools.com