Quixel updates its suite of advanced creation of textures and materials that includes dDO, nDO, 3DO and Megascans. 
The release of the new version (currently in pre-order) is scheduled for Q2/2014, and for the occasion, Quixel has decided to free the "old" version of DDO and NDO, guaranteeing the free upgrade to those who already owns the software. 
The current version of dDO can be downloaded at this link by clicking on Get dDO legacy, while at this link, you can always find nDO free download.