Not a flat you live inside, like in the UK – but a flat piece of cardboard.
Source Appleshadow
Submitted as a black pattern, I made it light and a few steps more subtle.
Source Andy
Seamless Prismatic Pythagorean Line Art Pattern No Background. A seamless pattern that includes the original tile (go to Objects / Pattern / Pattern To Objects in Inkscape's menu to extract it).
Source GDJ
This is a grid, only it’s noisy. You know. Reminds you of those printed grids you draw on.
Source Vectorpile
Greyscale version of a pattern that came out of playing with the 'light rays' plug-in for Paint.net
Source Firkin
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
Remixed from a drawing in 'Incidents on a Journey through Nubia to Darfoor', F. Ensor, 1891.
Source Firkin
The tile this is based on can be retrieved by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Adapted heavily from a JPG that was uploaded to Pixabay by Viscious-Speed.
Source Firkin
This ons is quite old school looking. Retro, even. I like it.
Source Arno Declercq
A pattern derived from part of a fractal rendering in Paint.net.
Source Firkin
Remixed from a drawing in 'Maidenhood; or, the Verge of the Stream', Laura Jewry, 1876.
Source Firkin
Bit of a strange name on this one, but still nice. Tiny gray square things.
Source Carlos Valdez
This one is amazing, truly original. Go use it!
Source Viahorizon
Light square grid pattern, great for a “DIY projects” sort of website, maybe?
Source Rafael Almeida
From a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Prismatic Hypnotic Pattern 2 No Background
Source GDJ
Prismatic Groovy Concentric Background
Source GDJ