Dvortype is a typing tutor program for Linux/Gtk+ 2.
It uses the ABCD Dvorak courses,
and is itself licensed under the GPL. When you download and extract it, run
./configure and make from its directory (and read the README).
Then type ./dvortype to start the program.
Just for fun, I made another version in python (using Tkinter). It's pretty similar, but this easily works in Windows.