Don't let the name fool you, MathGames.com offers more than just a series of math practice games. You can find hundreds of worksheets to print for free on MathGames.com. Those are organized according to grade level.
There are plenty of games for students to play on the site too. You can find those by clicking on the "games" header in the site. If you do that, scroll down the page a few times to find the MathGames.com digital textbook which organizes the games according to topic.
CK-12 Elementary Math Resources
CK-12 offers a good collection of resources for elementary school math practice. The collection is organized by grade level (grades 1 through 5) and skill set. The resources include a mix of videos and online practice exercises. Students can review a video and then attempt the practice activities.
MathQuiz.io is a math game developed by a student. It's a relatively simple site that presents you with a series of math problems to solve in your head then enter an answer. The problems are presented in sets of ten consecutive questions. You can play in an "easy" mode which is mostly simple addition, subtraction, and multiplication or you can plan in the "medium" mode which incorporates problems with fractions, division, and negative numbers.
A Maths Dictionary for Kids
Jenny Eather's A Maths Dictionary for Kids has been one of my go-to math resources for many years. It students provides simple and clear definitions of math terms. Each definition includes a small diagram or simple activity to illustrate the term's definition.
A Maths Dictionary for Kids has more than 270 free worksheets arranged according to topic. All of the worksheets can be found here.
ABCya offers hundreds of educational games for K-8 students. The site is arranged according to grade level and Common Core standard. The trouble with that arrangement is that you can't search for a skill without knowing the corresponding Common Core standard. If you use keyword search on the site, it will yield results to everything on the site, not just the games.