Some of the textures are non-free - a list is here http://ufoai.org/licenses/
We are working on replacing them with free versions.

Textures from http://www.mayang.com/textures/
* "License: These textures are Copyright \xa9 Mayang Murni Adnin, 2001-2006.
  They have all been taken by our own cameras or created by ourselves"
* "Use: They are free to use for all uses, whether commercial or non-commercial
  (you may incorporate them into your work with no requirement to pay us any
  licensing fee etc). It would be nice if you could credit us."
* "Sell: You may not sell any of these textures."
* "Redistribute: You may redistribute (give/send to somebody else/display on
  a web site) up to 30 of these textures together, but no more. You must credit
  this website as the original source (tell them about us)."

Textures from http://www.cgtextures.com
* "License: You may not sell or distribute any of these textures, use of these
  textures is only allowed under the following conditions:
  * Private or commercial use in 2D or 3D computer graphics and printed media.
  * Incorporation in computer games, 3D models.
  * Selling 3D models bundled with modified versions of the textures is allowed.
  * It is not allowed to sell or distribute any of these textures (either separate
    or in the form of texture packs.)"