I use this website as a quick check for understanding or for skill building. You can design a quiz yourself by writing your own questions, or you can choose questions that have already been written. There is a way to post the assignment to google classroom so that students just have to click the link. It will grade the quiz for you and you can opt to have students retake the quiz a certain number of times or infinitely many times.
I have students do it over and over again until they get 100%, but I don't count the assignment as a quiz, I count it as classwork. This is good for fast-thinking math questions they can do in their heads or on a piece of scratch paper quickly.
This is like the Teachers-Pay-Teachers website, BUT the activities on here have data to support that they are effective in learning. A lot of research has been put into the activities on this site and some of them are FREE!! They only cost a couple of dollars though if you do have to buy one.
I have used some of the free ones on here to help remediate or reteach. In particular, I used it this year in my trigonometry unit with students who were just not quite getting it. It worked really well as I saw great improvement in my low students!
This website is great for finding worksheets to have your students practice skills. Need them to practice graphing a line? Or solving two-step equations? This website has TONS of FREE worksheets for your students to use. They come with answer sheets and you can buy an online subscription if you want and you can make your worksheets digital.
I use these all the time when we need to practice skills. I leave the answer key up front for students to check the answers themselves as a way for them to self-assess their skills. They are required to show their work since they have access to the answers.