I have been getting a daily e-mail from Japanese Pod, a study website. I have not paid any money, or even given my credit card number, to the website. (They request the number, but I am not giving it.)
Anyway, these daily emails are useful. At first, I thought they were too easy. But then I realized that there are sentences which accompany the vocabulary word, and so even if the new word of the day is too easy for me, I can often learn a new word from the example sentences.