I just thought I would post this, as it needs some testers.
Linuxcnc running on the raspberry pi maybe is getting very close to a usable high speed motion controller.
There are 22 IO pins available for running step/dir motor drivers, real time kernel and almost all of the linuxcnc components running correctly.
If you can test this and give feedback to the linuxcnc.org forum it will help speed up the development.
It does not need any extra hardware just plug your stepper drivers into the relevant gpio header pins (thought really I would like people with scopes to send in signal plots and test functions, so plugging motors is not recomended).