I mainly use textures from

http://www.vb3d.com/Textures.html

Many high quality, often tileable textures. Originally developed by Crack.Com company (authors of "Abuse"). These textures were intended to be used in their next game "Golgotha", but company went bankrupt and they just published these textures. License is public domain (i.e. no license), so no use restrictions. The same textures are also hosted on some other sites, like

http://www.wolfiesden.com/golgotha/golgotha.asp