For experimental purposes you can grab YAFFS free of charge right now from our GIT repository.
If you distribute a product using YAFFS you must release all your linked source code in accordance with the GPL. Alternatively if you do not wish to disclose your Source Code see the FAQs and this discussion.
YAFFS Licences are non-exclusive, perpetual, world-wide, royalty-free and paid for with a single fee for one product. A Licence entitles you to most further development and improvement of the YAFFS code.
You can find further information about each licence in these documents: