A comeback for you: the popular Escheresque, now in black.
Source Patten
A frame using leaves from an image that was uploaded to Pixabay by mayapujiati
Source Firkin
Remixed from an image that was uploaded to Pixabay by Pixeline
Source Firkin
Sharp pixel pattern, just like the good old days.
Source Paridhi
A seamless pattern the starting point for which was a 'rainbow twist' texture in Paint.net.
Source Firkin
And some more testing, this time with Seamless Studio. It’s Robots FFS!
Source Seamless Studio
A seamless pattern formed from a square tile. The tile can be retrieved by selecting the rectangle in Inkscape and using shift-alt-i
Source Firkin
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
This was formed by distorting an image of a background on Pixabay.
Source Firkin
A seamless pattern the unit cell for which can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin
Background formed from the iconic plastic construction bricks that gave me endless hours of fun when I was a lad.
Source Firkin
Seamless , tileable CC-0 texture. Created by my own, feel free to use wherever you want!
Source Linolafett
Looks as if it's spray painted on the wall. You can be sure that this pattern will seamlessly fill your backgrounds on web pages.
Source V. Hartikainen
A pale orange background pattern with glossy groove stripes.
Source V. Hartikainen
I know there is one here already, but this is sexy!
Source Gjermund Gustavsen
Seamless pattern the tile for which can be had by using shift-alt-I on the selected rectangle in Inkscape.
Source Firkin
From a drawing of the coat of arms of the Ottoman Empire on Wikimedia.
Source Firkin
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
You can never get enough of these tiny pixel patterns with sharp lines.
Source Designova
Remixed from a design seen on Pixabay. The basic tile can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin
Original seamless pattern with an Inkscape filter.
Source Firkin
Not a flat you live inside, like in the UK – but a flat piece of cardboard.
Source Appleshadow