BrainBashers has a huge collection of games, quizzes, puzzles, riddles, illusions, word searches and more. The riddles and word puzzles are probably the best way to practice English, but the Sudoku-type puzzles and illusions will also give your brain some exercise. Some of the puzzles require flash.
Here’s the most popular puzzle on the site (as of today). It was supposedly written by Albert Einstein. Can you figure it out?