My son is using the math flashcards for Geometry this year. Last year he used them for Algebra and Latin. My older daughter uses the SAT vocabulary lists. My youngest daughter uses it for science. SpellingCity has vocabulary lists for books. If I can't find one for the book she's about to read, then I create a list for her. This helps her with reading comprehension as she reads the book because she's already familiar with the words that would otherwise be new to her. Because she's been practicing these words with the flashcards, she knows the definitions.
There are vocabulary games or parts of speech games online that my kids often use and enjoy, but the advantage to the SpellingCity flashcards is that we can create our own vocabulary lists. This is particularly helpful when studying for unit tests, a particular lesson or preparing to read a selected book.
What do you use to study vocabulary? What works best with your children?