I have used Creative Commons licenses in the past. As a musician I tend to release material under the CC-BY-NC license. If anyone happens to want to use my music in a commercial setting (and they do) there is a means for them to pay for that. Everyone else is free to download the music for free, adapt it and even give it away to friends for no cost beyond the bandwidth to download.

For any education materials I may produce in the future I would choose the same (CC-BY-NC) license.  It seems fair that educators / learners can use the material for no cost if they are not profiting from it (for instance by repackaging and selling it). 

The BY part is a useful addition – as the material is adapted and passed on all authors are recognised for their contribution (as well as providing an informal audit trail for the contents).