Dare I call this a «flat pattern»? Probably not.
Source Dax Kieran
From a drawing in 'Cowdray: the history of a great English House', Julia Roundell, 1884.
Source Firkin
A comeback for you: the popular Escheresque, now in black.
Source Patten
The name alone is awesome, but so is this sweet dark pattern.
Source Federica Pelzel
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
This background has abstract texture with some similarities to wood.
Source V. Hartikainen
A beautiful dark padded pattern, like an old classic sofa.
Source Chris Baldie
To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Background formed from the original with an emboss effect
Source GDJ
A playful triangle pattern with different shades of gray.
Source Dimitrie Hoekstra
With a name like this, it has to be hot. Diagonal lines in light shades.
Source Isaac
This was formed by distorting an image of a background on Pixabay.
Source Firkin
A seamless green background texture. The image is distributed under a Creative Commons License (like all of the images here).
Source V. Hartikainen
The classic subtle pattern. Sort of wall/brick looking. Or moon-looking?
Source Joel Klein
As simple and subtle as it gets. But sometimes that’s just what you want.
Source Designova
CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
You know I love paper patterns. Here is one from Stephen. Say thank you!
Source Stephen Gilbert
This makes me wanna shoot some pool! Sweet green pool table pattern.
Source Caveman
A version without colours blended together to give a different look.
Source Firkin
This one needs to be used in small areas; you can see it repeat.
Source Luca
From a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin