Free Vocabulary Resources
English has about half a million words in it: more than any other language! We only use about a thousand of them in everyday spoken communication, and about 10,000 more in most writing. It is important that children be exposed to vocabulary at a young age, so that when they come across the 9,900 words that they're unfamiliar with from conversational spoken language, they understand what they are reading.
Free Word Study Resources
Merriam-Webster publishes a list of 8 vocabulary words a week, with their definitions. There are a mix of levels in here: some words I'd consider middle school vocabulary, others would be more appropriate for younger students. Still, a great review of a variety of words.
The website ExamWords.com has a whole bunch of lists of vocabulary and spelling words, for students from elementary school all the way up to high school. This is a great place to find words that might appear on the SATs.
Vocabulary.com offers similar lists of words, some of them in convenient "flashcard" format.