I stumbled across this free app, Math Planet, when I was looking for free apps to use next year. Although, this is labeled middle school but I think it could easily be used in a Pre-algebra or Algebra class who needs additional scaffolding. It is a series of games that are set up so the student cannot move on unless they score a minimum number. Each level is a game that tests something different. Some activities would be quite easy for 8th and 9th graders but others do take a certain amount of skill. There was a game that required me to put numbers on a number line in the correct order and in the correct location so I could drop bombs on the monster and the game gave an accuracy rating. Apparently there are over 100 games in it. I think I got as far as game 6 on level 1. I was shooting asteroids at a certain angle. I get points for managing to hit the asteroid. It was actually quite fun.
Each skill that is tested is done as a game. Student engagement will depend on the level of their understanding and ability for the skill but I think every student will find something they will enjoy.