Math Planet is a free online resource that allows students to learn math concepts. Pre-algebra, Algebra 1, Algebra 2, and Geometry are some of the high school math courses available. In addition, they have developed practice tests for the SAT and ACT exams.
The teaching content is geared toward high school mathematics in the United States. However, because mathematics is the same everywhere globally, you are welcome to come and learn mathematics with them for free.
If you need a fun math tool for your students, you may want to visit Can You Draw a Perfect Circle!