CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
Just to prove my point, here is a slightly modified dark version.
Source Atle Mo
Adapted heavily from a JPG that was uploaded to Pixabay by Viscious-Speed.
Source Firkin
Drawn in Paint.net and vectorised in Vector Magic.
Source Firkin
This one needs to be used in small areas; you can see it repeat.
Source Luca
Made by distorting a simple pattern using the 'sin waves' plugin for Paint.net and vectorising in Vector Magic
Source Firkin
Colour version that is close to the original drawing uploaded to Pixabay by pencilparker.
Source Firkin
This is a hot one. Small, sharp and unique.
Source GraphicsWall
Lovely pattern with splattered vintage speckles.
Source David Pomfret
Sharp pixel pattern looking like some sort of fabric.
Source Dmitry
An emulated “transparent” background pattern, like that of all kinds of computer graphics software.
Source AdamStanislav
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
You were craving more leather, so I whipped this up by scanning a leather jacket.
Source Atle Mo
Abstract Tiled Background Extended 6
Source GDJ
This is a seamless pattern of regular hexagon which has a honeycomb structure.
Source Yamachem
From a drawing in 'Art Embroidery', M.S. Lockwood and E. Glaister, 1878.
Source Firkin
Alternative colour scheme. 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 'Gately's World's Progress', Charles Beale, 1886.
Source Firkin