Mavis Beacon without a doubt, is the most popular typing package in the world. However, a less known but better alternative to Mavis Beacon is Tux Typing. This cross platform open source typing application is designed to be intuitive, fun and educative with a special emphasis on children. It features the ever funny Tux-the official mascot of Linux- as the tutor.
Penguins are know to like fish, thus in the lessons of the application, you help Tux eat falling fishes by typing on your keyboard the corresponding letters on the fishes. You type something wrong and Tuxy will miss the fish to the ground. You can also write your own lessons as part of the teaching package. Tux Typing was designed with kids in mind but has increasingly difficult levels that even experienced typists will find challenging.
It is available for download for all platforms. Ubuntu users should just search for Tux Typing in the Ubuntu Software Center or issue sudo apt-get install tuxtype at the terminal. It is also available in the repos of most of the distros out there.