Medium gray fabric pattern with 45-degree lines going across.
Source Atle Mo
A seamless chequerboard pattern formed from a tile that can be had in Inkscape by selecting the rectangle and using shift+alt+i. Alternative colour scheme.
Source Firkin
Prismatic Curved Diamond Pattern 3 No Background
Source GDJ
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
Formed from decorative divider 184 in paint.net. Vectorised with Vector Magic.
Source Firkin
Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
This one could be the shirt of a golf player. Angled lines in different thicknesses.
Source Olivier Pineda
The rectangular tile this is based on can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin
This is so subtle I hope you can see it! Tweak at will.
Source Alexandre Naud
Almost like little fish shells, or dragon skin.
Source Graphiste
It has waves, so make sure you don’t get sea sickness.
Source CoolPatterns
Could remind you a bit of those squares in Super Mario Bros, yeh?
Source Jeff Wall
A seamless pattern drawn originally in Paint.net by distorting a slice of background pattern 116 and copying the resulting triangle numerous times.
Source Firkin
One more brick pattern. A bit more depth to this one.
Source Benjamin Ward
Seamless , tileable CC-0 texture. Created by my own, feel free to use wherever you want!
Source Linolafett
The square tile this is based on can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin
Bright Multicolored Floral Background by Karen Arnold from PDP.
Source GDJ