These are resources, not necessarily recommendations (though for some tools I've written a glowing review). Some are free and others are free to try then require purchase. Please check them out and decide for yourself if they will work in your classroom.
Free Math Apps by topic
Prodigy - game-based learning; the math concepts presented are based on a pre-assessment. Good for students who finish classwork early, or for students who need extra practice
Open Middle - leaves part of an equation blank - students need to fill in the “middle” to make it true
Would You Rather Math - link is for grades 6-8 - students can discuss which is the better of two options using math concepts
Which One Doesn’t Belong - reasoning and logic based on math concepts
Fraction Talks - to visualize
ReadWorldMath: uses GoogleEarth to teach practical application of math concepts (may not work great on Chromebooks 😞
Cuethink: Math problem solving and collaboration skills