WordBit is a free app that minimizes the effort and thought that goes into deliberate practice by presenting you with the opportunity to practice each time you open your lock screen.
Each time you open your phone, the app will overlay the lock screen and present you with either multiple choice translations for a given word, or a flashcard. You can choose to close the app to access the lock screen, or respond to the prompt. Although this app interferes with tasks on your phone that require immediate attention, it is no doubt effective at consistently exposing you to new vocabulary.
It is available in multiple languages, for both target and source languages, and there is a large vocabulary category bank to choose from, including vocabulary from each of the levels A1-C2.
There are some bugs in its programming, such as restarting your progress through each deck if you add or remove a category, and the ads at the bottom of the screen are easily tapped by accident.