Not a flat you live inside, like in the UK – but a flat piece of cardboard.
Source Appleshadow
This is a hot one. Small, sharp and unique.
Source GraphicsWall
From a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Subtle scratches on a light gray background.
Source Andrey Ovcharov
Prismatic Geometric Pattern Background
Source GDJ
To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Not a pattern for fabrics, but one produced from a jpg of a stack of fabric items that was posted on Pixabay. The tile that this is based on can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Sort of like the Photoshop transparent background, but better!
Source Alex Parker
Zero CC tileable hard cover cells book texture, 4k, scanned and made by me. CC0
Source Sojan Janso
Background Wall, Art Abstract, Blue Well & CC0 texture.
Source Ractapopulous
A free black metallic background pattern. Here's a new pattern I made that looks metallic.
Source V. Hartikainen
Sharp diamond pattern. A small 24x18px tile.
Source Tom Neal
Bright Multicolored Floral Background by Karen Arnold from PDP.
Source GDJ
Used a cherry by doctormo to make this seamless pattern
Source Firkin
Seamless pattern the basic tile for which can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Very dark pattern with some noise and 45-degree lines.
Source Stefan Aleksić
Zero CC tileable hard cover red book, scanned and made by me. CC0
Source Sojan Janso
Looks a bit like concrete with subtle specks spread around the pattern.
Source Mladjan Antic