Vocabulary games can help you learn new words and the meaning of those words. A large vocabulary generally equates to better reading comprehension. Vocabulary skills are crucial to everyone, not just kids in elementary school. If you are practicing for the SAT, GRE, GED, or your third grade vocabulary test, building a good vocabulary will take you much farther down the road.
Kids like playing games, so playing vocabulary games will most likely encourage your child to work on their vocabulary skills. The games help keep kids interested, and the kids tend to retain what they learn much better.
Vocabulary Fun is a great site that offers online flash games to help you build reading, phonics, and English language skills. The site is free to use.