There is a program that is opensource (therefore free in its "full" sense).

It's called dmak. It's both for linux and windows.
I actually tried it on linux, but it didn't work out well.

Well on windows it worked. If you are interested search it, maybe words like "dmak" and "opensource". If it's open source, it's "automatically" free to use copy and modify.
(just wanted to share... I use it)