CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
A seamless pattern the starting point for which was a 'light rays' rendering in Paint.net.
Source Firkin
There are many carbon patterns, but this one is tiny.
Source Designova
Abstract Geometric Monochrome Pattern Prismatic No Background
Source GDJ
From a drawing in 'Line and form', Walter Crane, 1914.
Source Firkin
To get the tile this is based on select the rectangle in Inkscape and use shift-alt-i
Source Firkin
This was formed by distorting an image of a background on Pixabay.
Source Firkin
Prismatic Curved Diamond Pattern 7 No Background
Source GDJ
A slightly grainy paper pattern with small horizontal and vertical strokes.
Source Atle Mo
Detailed but still subtle and quite original. Lovely gray shades.
Source Kim Ruddock
Seamless pattern formed from a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin
To get the tile this is based on, select the rectangle in Inkscape and use shift-alt-i.
Source Firkin
Semi-light fabric pattern made out of random pixels in shades of gray.
Source Atle Mo
Prismatic Polka Dots 3 No Background
Source GDJ
I took the liberty of using Dmitry’s pattern and made a version without perforation.
Source Atle Mo
Remixed from a design on Pixabay. To get the tile this is based on select the rectangle in Inkscape and use shift-alt-i.
Source Firkin
Here's a seamless brown cork board background texture. Feel free to download or reshare if you like.
Source V. Hartikainen
A simple circle. That’s all it takes. This one is even transparent, for those who like that.
Source Saqib
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
Actually, there's no clouds in it, but I think it looks quite nice.
Source V. Hartikainen
A monochrome pattern from a tile that can be had by selecting the rectangle in Inkscaope and using shift+alt+i
Source Firkin
Remixed from a drawing in 'Chambéry à la fin du XIVe siècle', Timoleon Chapperon, 1863.
Source Firkin
This is so subtle you need to bring your magnifier!
Source Carlos Valdez