The BBC Learning English site has such a wealth of resources that it's easy to overlook some of the hidden gems. As case in point is the excellent Wordmaster game, which is buried away in the Quizzes section. It's a bit like hangman without the gallows. A nice touch is that you can have a definition of the word you're trying to find (although doing so will lose you some points). There are thousands of words to practise so this is a really good way to improve your English vocabulary.