I found a new seamless texture Website that I want to share. I could have sworn there was a thread for this somewhere, but for the life of me I can't find it. Please feel free to move this post if such a thread already exists, if not, please feel free to add links to this one.

Artist David Gurrea has hundreds of scalable tiles on his Web site perfect for mapping, most of which are 1000 x 1000 pixels:
http://www.davegh.com/blade/davegh.htm

And some more of my favorites are linked below...

Absolute Background Textures Archive:
http://www.grsites.com/textures/

Paul Bourke's Textures:
http://local.wasp.uwa.edu.au/~pbourke/texture_colour/

Blu's Cave:
http://www.bluscave.com/bitmap_fills.htm

Neotox:
http://www.neotox.com/stock-stone.htm

ScentedNectar:
http://www.scentednectar.com/textures/index.htm

Renfield Imaging:
WARNING! Make sure you have a pop-up blocker activated in your Internet browser before you click on the link below!
http://www.renfield-imaging.50megs.com/index.html