If you've been meaning to get around to working on your lacklustre keyboard chops Klava can help you hone your touch typing skills without having to install any apps or sign up for any services.
The flash-based keyboard tutor has a simple interface. You can select from basic English, advanced English — the change is just the length and complexity of the words — as well as programming languages and if you're learning a bit of Russian, Cyrillic languages. You can use the QWERTY, Dvorak or Cyrillic keyboard arrangement: the on-screen keyboard will change, but it's up to you to configure your system to use that keyboard arrangement.
Unlike many typing tutors you won't find a countdown clock, you just start typing and it begins counting your words per minute and word accuracy. When you make a typo the text box turns dark grey and remains that way until you correct the error. Klava is a free service and requires no sign up. Have a favourite typing tutor? Let's hear about it in the comments.