Khan Academy is a fantastic resource for any age and grade level. This website covers math from pre-k through college, and has other subjects as well. Each lesson includes clear, in-depth explanations of topics as well as practice problems. This is a great tool for reinforcing and practicing concepts learned in class.
The language learning app Duolingo also has math! This is a great tool for students who struggle with fundamental concepts like multiplication, fractions, place value, etc. Strengthening those fundamentals will have a positive effect on confidence in high school math!
eMATHinstruction is a company that creates math curriculum. They also have a YouTube account full of video lessons for 6th grade math through Algebra II. Perhaps my favorite part of this channel is their archive of regents review livestreams. Every June, they hold live regents review sessions the night before each regents exam.
We use the TI-84 Plus graphing calculators in class, and they can be challenging to master. Texas Instruments, the manufacturer, has a YouTube channel full of tutorials on how to use their graphing calculator in all different classes. There are the basic function tutorials to get you started, but there is so much more. They have test success and digital exam videos, and even do livestreams for students taking precalculus and above.