A repeating gloomy background image. This one consists of a pattern of black chains layered on top of a dark textured background.
Source V. Hartikainen
A seamless chequerboard pattern formed from a tile that can be had in Inkscape by selecting the rectangle and using shift+alt+i.
Source Firkin
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
Dark squares with some virus-looking dots in the grid.
Source Hugo Loning
The green fibers pattern will work very well in grayscale as well.
Source Matteo Di Capua
The image depicts a Japanese Edo pattern called "kanoko or 鹿の子" meaning "fawn" which has a fur with small white spots.
Source Yamachem
Inspired by the B&O Play, I had to make this pattern.
Source Atle Mo
To get the tile this is based on select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Produced using the clouds, flames and glass blocks plug-ins in Paint.net and the resulting .PNG vectorised with Vector Magic.
Source Firkin
Here I have tried to create something that would look like maple wood. Not sure how well it's turned out, but at least it looks like wood.
Source V. Hartikainen
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
A slightly more textured pattern, medium gray. A bit like a potato sack?
Source Bilal Ketab
The image depicts a seamless pattern of a fishnet with a plenty of fish.It may be a lucky charm for fishermen.
Source Yamachem
A smooth mid-tone gray, or low contrast if you will, linen pattern.
Source Jordan Pittman
Non-seamless pattern drawn in Paint.net and vectorised in Vector Magic.
Source Firkin
The act or state of corrugating or of being corrugated, a wrinkle; fold; furrow; ridge.
Source Anna Litvinuk
If you like it a bit trippy, this wave pattern might be for you.
Source Ian Soper
From a drawing in 'Art Embroidery', M.S. Lockwood and E. Glaister, 1878.
Source Firkin
CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
A bit like some carbon, or knitted netting if you will.
Source Anna Litvinuk
Remixed from a drawing that was uploaded to Pixabay by ractapopulous
Source Firkin
A rusty grunge background for websites. Feel free to use it in your site's theme.
Source V. Hartikainen