CC0 and seamless wellington boot pattern.
Source SliverKnight
To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Looks like a technical drawing board: small squares forming a nice grid.
Source We Are Pixel8
A background pattern inspired by designs seen in 'Burghley. The Life of William Cecil', William Charlton, 1857.
Source Firkin
A seamless background drawn in Paint.net and vectorised with Vector Magic. The starting point was a photograph of drinking straws from Pixabay.
Source Firkin
A repeating background for websites with a texture of black groove stripes.
Source V. Hartikainen
A nice looking light gray background pattern with diagonal stripes.
Source V. Hartikainen
A seamless pattern from a tile drawn in Paint.net and vectorised in Vector Magic
Source Firkin
The green fibers pattern will work very well in grayscale as well.
Source Matteo Di Capua
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
This one takes you back to math class. Classic mathematic board underlay.
Source Josh Green
Sometimes you just need the simplest thing.
Source Fabricio
Prismatic Groovy Concentric Background 4 No Black
Source GDJ
This pattern comes in orange, and it looks as if it is "made of glass".
Source V. Hartikainen
A seamless pattern created from a square tile. To get the tile, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
A beautiful dark padded pattern, like an old classic sofa.
Source Chris Baldie
A good starting point for a cardboard pattern. This would work well in a variety of colors.
Source Atle Mo
Design drawn in Paint.net, vectorised using Vector Magic and finished in Inkscape.
Source Firkin
Pattern Background, Texture, Photoshop Structure style CC0 texture.
Source Darkmoon1968
A seamless pattern the unit cell for which can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin