There are two schools of thought that think of typing as an actual skill that needs to be taught and refined and those that believe it is just something that evolves as your computer skills develop in general.  I am not so phased with typing as I probably should be but I have come across a few worthwhile sites that I will direct my students to have a play on to hopefully increase their skills on the keyboard in their own time.  Enjoy and I'd love to hear of any others you know of.

Bubbles Typing Game: 
Push your typing skills to their limits by playing the popular Bubbles typing game. In the game your goal is to burst the bubbles by pressing the corresponding key.

Free Typing Game:  A mix of typing games students will enjoy

Keybr.com - A great little flash based tutorial site for the more advanced typer.

10 Finger Breakout:  10 Finger BreakOut is a REAL arcade game, in which you are escaping from invaders, shooting and trying to hit balls. This is no fake - you are playing BreakOut. (You are not just typing characters). With this free typing game you will have fun, but don't worry, by playing 10 Finger BreakOut you sure will be learning to type. Improve your typing skills with this free typing tutor - typing game.( you will need to download and install )

BBC Dance Mat Typing:  Meet the gang and learn to type.  Make your way through all 12 stages and you can be a top typist.