Not a pattern for fabrics, but one produced from a jpg of a stack of fabric items that was posted on Pixabay. The tile that this is based on can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
From a drawing in 'Jardyne's Wife', Charles Wills, 1891.
Source Firkin
I have no idea what J Boo means by this name, but hey – it’s hot.
Source j Boo
You don’t see many mid-tone patterns here, but this one is nice.
Source Joel Klein
Prismatic Abstract Line Art Pattern Background
Source GDJ
Classic vertical lines, in all its subtlety.
Source Cody L
Spice up your next school project with this icon background.
Source Swetha
Classic 45-degree pattern, light version.
Source Luke McDonald
A new take on the black linen pattern. Softer this time.
Source Atle Mo
Colour version of the original seamless pattern.
Source Firkin
This reminds me of Game Cube. A nice light 3D cube pattern.
Source Sander Ottens
It’s an egg, in the form of a pattern. This really is 2012.
Source Paul Phönixweiß
Looks like an old wall. I guess that’s it then?
Source Viahorizon
I know there is one here already, but this is sexy!
Source Gjermund Gustavsen
Seamless , tileable CC-0 texture. Created by my own, feel free to use wherever you want!
Source Linolafett
Sharp pixel pattern looking like some sort of fabric.
Source Dmitry
From a drawing in 'Resa i Afrika, genom Angola, Ovampo och Damaraland', P. Moller, 1899.
Source Firkin
A seamless pattern based on a rectangular tile that can be retrieved in Inkscape by selecting the rectangle and using shift-alt-i.
Source Firkin
Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
A playful triangle pattern with different shades of gray.
Source Dimitrie Hoekstra
To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Dark, crisp and subtle. Tiny black lines on top of some noise.
Source Wilmotte Bastien