Vocup is a free vocabulary trainer.
At first you have to add your words to your digital vocabulary book. After this, Vocup will ask you every word and repeat it if you made a mistake. You can easily share your vocabulary books with friends who use Vocup too.
Florian Amstutz initially developed Vocup and discontinued support in 2018. Since then Daniel Lerch continued developing Vocup open source on GitHub: https://github.com/daniel-lerch/vocup