A few months ago I started using Anki to fill the massive hole in my knowledge of German vocabulary. I've since studied more than 1600 flashcards and it's helped me immensely. Anki will automatically space out repetitions of the cards for you depending how difficult or easy you found them. This is called spaced repetition and is an evidence based way to remember things in the long term. Just make sure to force yourself to do it every day.
https://apps.ankiweb.net/ It's free and open source
For German learners, here is the deck that's helped me the most: https://ankiweb.net/shared/info/653061995
4000 words may seem like a lot but it's worth it. Every word has an example sentence, audio sample, as well as plural forms if a noun and for the verbs some conjugated forms and simple past and past participle.
also it's good for remembering other shit too. med students use it religiously
Anki is definitely useful, I did it for French for a while. My only problem is it got really clogged with words for a while and it made it harder to learn. I think I might have been using it wrong though, like it would ask me how many days I wanted it to be before it showed me the card again, or if I wanted to see it again the same day and I was never sure what was the best choice, I am not qualified to make that decision lol.
On mine i turned off the showing the days. So it just says, don't know, hard, good, and easy.
Makes it simple for me to just answer honestly how i felt about each card and not worry about the days
I'll look for a setting for that if I try again