Not a flat you live inside, like in the UK – but a flat piece of cardboard.
Source Appleshadow
One can never have too few rice paper patterns, so here is one more.
Source Atle Mo
A seamless pattern from a tile drawn in Paint.net and vectorised in Vector Magic
Source Firkin
Small gradient crosses inside 45-degree boxes, or bigger crosses if you will.
Source Wassim
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
Uses spirals from Pixabay. To get the basic tile select the rectangle in Inkscape and use shift+alt+i
Source Firkin
A pattern formed from a photograph of a 16th century ceramic tile.
Source Firkin
A pattern formed from a squared tile. The tile can be accessed in Inkscape by selecting the rectangle and using shift-alt-i.
Source Firkin
Fake or not, it’s quite luxurious.
Source Factorio.us Collective
A free background pattern with abstract green tiles.
Source V. Hartikainen
Prismatic Hexagonalist Pattern No Background
Source GDJ
Wild Oliva or Oliva Wilde? Darker than the others, sort of a medium dark pattern.
Source Badhon Ebrahim