Math Galaxy 8th Grade is a comprehensive and very useful review and practice app that covers all of the math standards from Common Core in this grade. It offers quick, clear review text and videos and an infinite set of practice problems for each skill.
Math Climber HD is an outstanding, engaging resource for practicing basic math and prealgebra skills, useful for students who are learning these skills for the first time and those who need quick review or extra practice.
The free version of Mathway provides students with ready answers to math and chemistry problems at nearly any level. A subscription must be purchased to access step-by-step solutions that some students may need to understand the process of solving the problems, though.
Monkey Party Learn Algebra makes an entertaining and valuable introduction to basic early algebra concepts, including balancing equations, combining like terms, using negative numbers, multiplying with fractions, and introducing the concept of graphing lines based on equations.