I am not a huge fan of flashcard learning, but as a teacher it is a useful tool, especially with younger learners. It certainly has its place to bolster learning, if you are forgetful or want to cram a lot of new words in for a particular subject. I have been away from this site a fair while so forgive me if MEMrise has already been suggested but I just thought it was worth a little praise for its intermediate plus content, I find personally I am past the beginner stage and it becomes quite tricky to source content that is suitable for my level or sites where you don;t have to start from scratch and cover the things you already know, which can be a real bore!
https://app.memrise.com/bienvenue?source_language=en&difficulty=2022184&target_language=es#1