A pattern derived from part of a fractal rendering in Paint.net.
Source Firkin
One more from Badhon, sharp horizontal lines making an embossed paper feeling.
Source Badhon Ebrahim
Formed from a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Derived from an image that was uploaded to Pixabay by starchim01
Source Firkin
CC0 remixed from a drawing. Walter Crane, 1914, Firkin.
Source SliverKnight
Not a flat you live inside, like in the UK – but a flat piece of cardboard.
Source Appleshadow
This one could be the shirt of a golf player. Angled lines in different thicknesses.
Source Olivier Pineda
Seamless pattern formed from a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin
A free seamless background image with a texture of dark red "canvas". It should look very nice on web sites.
Source V. Hartikainen
Seamless pattern the basic tile for which can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
An orange vertically striped background pattern. Feel free to download and use this orange background pattern, for example, on the web). It resembles a wallpaper with vertical stripes or something similar to it.
Source V. Hartikainen
A free background image with a seamless texture of cardboard. This texture of cardboard looks quite realistic, especially when is actually tiled.
Source V. Hartikainen
This ons is quite old school looking. Retro, even. I like it.
Source Arno Declercq
Prismatic Geometric Pattern Background
Source GDJ
Pixel by pixel, sharp and clean. Very light pattern with clear lines.
Source M.Ashok
Submitted by DomainsInfo – wtf, right? But hey, a free pattern.
Source DomainsInfo
From a drawing in 'At home', J. Sowerby, J. Crane and T. Frederick, 1881.
Source Firkin
And some more testing, this time with Seamless Studio. It’s Robots FFS!
Source Seamless Studio
Zero CC tileable hard cover red book, scanned and made by me. CC0
Source Sojan Janso
Dark wooden pattern, given the subtle treatment. based on texture from Cloaks. https://cloaks.deviantart.com
Source Atle Mo
A comeback for you: the popular Escheresque, now in black.
Source Patten