People seem to enjoy dark patterns, so here is one with some circles.
Source Atle Mo
Snap! It’s a pattern, and it’s not grayscale! Of course you can always change the color in Photoshop.
Source Atle Mo
Prismatic Groovy Concentric Background 2
Source GDJ
I know there is one here already, but this is sexy!
Source Gjermund Gustavsen
From a drawing in 'Real Sailor-Songs', John Ashton, 1891.
Source Firkin
A pattern derived from repeating unit cells each derived from part of a fractal rendering in paint.net.
Source Firkin
Awesome name, great pattern. Who does not love space?
Source Nick Batchelor
To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
A background pattern with a look of rough fabric.
Source V. Hartikainen
Prismatic Groovy Concentric Background 3
Source GDJ
Sounds French. Some 3D square diagonals, that’s all you need to know.
Source Graphiste
This background has abstract texture with some similarities to wood.
Source V. Hartikainen
A heavy hitter at 400x400px, but lovely still.
Source Breezi
To get the tile this is based on select the rectangle in Inkscape and use shift-alt-i
Source Firkin
A very dark spotted twinkle pattern for your twinkle needs.
Source Badhon Ebrahim
The square tile this is based on can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin
Seamless pattern the basic tile for which can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Dark, crisp and subtle. Tiny black lines on top of some noise.
Source Wilmotte Bastien
A free seamless background image with abstract texture of green "curtain".
Source V. Hartikainen
Horizontal and vertical lines on a light gray background.
Source Adam Anlauf
As simple and subtle as it gets. But sometimes that’s just what you want.
Source Designova
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
A seamless pattern formed from a square tile. To get the tile, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
From a drawing in 'Uit de geschiedenis der Heilige Stede te Amsterdam', Yohannes Sterck, 1898.
Source Firkin