Knoword is a fast-paced word game that helps boost vocabulary, spelling, speed of thought, as well as increasing analytical, observational and typing skills. When you start a new game you'll be given a definition, the first letter of the word it's referring to, and 1 minute to solve the problem. Guess the correct word and you'll move on to the next puzzle. If you don't know the answer, simply press the "X"-shaped skip button. Every correct answer earns 20 points, and every skipped question loses you 10.
This game is seriously addictive, fun to play, and unlike a lot of similar games and apps which test SAT vocabulary and the like, the words and definitions are not too obscure. A particularly nice touch is the way the system uses colours to show whether you've typed a right or wrong letter, although it's frustrating not to get the answer to the word you're trying to find when you finally run out of time. There's no app, but the game works fine on an iPad as long as you're in portrait mode. You can choose which level to play at (Novice, Hotshot, or Wizard), but I do think learners below post-intermediate level would find the game challenging, even at the Novice level.
If you've got a videoprojector in the classroom, you could play Knoword as a team game with the whole class.