I've used dozens of textures from this site.
http://www.grsites.com/textures/
This has some interesting textures too
http://local.wasp.uwa.edu.au/~pbourke/t ... lour/dirt/
Valve Job once linked us up with a tutorial on how to make 'any' texture tile properly but I can't find it, or the site is no longer around. He used it to make lakebed extreme. I probably have the page saved some place but can't find it right now. Anyway, that link above has a good description of the process that we can work with using exerpts from that large image as the starting point. Cut out any square section then see here:
http://local.wasp.uwa.edu.au/~pbourke/t ... ur/tiling/
and obeone/rudee's photoshop methods
http://cownap.com/~mtmg/misc/obe/
the 'old' reptile program and trial-key (the 'new' sausage site is a mess)
http://mtm2.com/~mtmg/downloads/misc/paint/reptinst.exe
http://mtm2.com/~mtmg/downloads/misc/paint/reptinst.txt
and if you're writing a program...
http://local.wasp.uwa.edu.au/~pbourke/t ... nperiodic/