Kahoot
Who has never heard of Kahoot?! It is incredibly good, use it now. (Disclaimer: it is addictive)
Forest
Forest is an app that will encourage but not force you to work by creating a virtual forest. You set the time you want to stay focused and during that time the tree will grow. If you get distracted the tree will die. You can grow you own forest!
Quizlet
The best app to learn words! Quizlet is the best app to learn a vocabulary list, to revise tests. You should definitely check it out.