Dark squares with some virus-looking dots in the grid.
Source Hugo Loning
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 rusty grunge background for websites. Feel free to use it in your site's theme.
Source V. Hartikainen
And some more testing, this time with Seamless Studio. It’s Robots FFS!
Source Seamless Studio
A comeback for you: the popular Escheresque, now in black.
Source Patten
Seamless pattern formed from a tile that 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
The image is a seamless pattern of a fishnet.
Source Yamachem
Simple gray checkered lines, in light tones.
Source Radosław Rzepecki
You don’t see many mid-tone patterns here, but this one is nice.
Source Joel Klein
A seamless design of flowers remixed from a jpg on Pixabay by Prawny.
Source Firkin
Prismatic Geometric Pattern Background 2 No Black
Source GDJ
Prismatic Groovy Concentric Background 3
Source GDJ
To get the tile this is based on, select the rectangle in Inkscape and use shift-alt-i.
Source Firkin
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
Super subtle indeed, a medium gray pattern with tiny dots in a grid.
Source Designova
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
An alternative colour scheme to the original seamless pattern.
Source Firkin
Luxurious looking pattern (for a T-shirt maybe?) with a hint of green.
Source Simon Meek
ZeroCC tileable stone texture, edited from pixabay. CC0
Source Sojan Janso
I took the liberty of using Dmitry’s pattern and made a version without perforation.
Source Atle Mo
Small gradient crosses inside 45-degree boxes, or bigger crosses if you will.
Source Wassim