Not a flat you live inside, like in the UK – but a flat piece of cardboard.
Source Appleshadow
Abstract Ellipses Background Grayscale
Source GDJ
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
From a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Some rectangles, a bit of dust and grunge, plus a hint of concrete.
Source Atle Mo
Retro Circles Background 4 No Black
Source GDJ
Light gray version of the Binding pattern that looks a bit like fabric.
Source Newbury
I know there is one here already, but this is sexy!
Source Gjermund Gustavsen
Imagine you zoomed in 1000X on some fabric. But then it turned out to be a skeleton!
Source Angelica
Remixed from a drawing in 'Works. Popular edition', John Ruskin, 1886.
Source Firkin
Paper pattern with small dust particles and 45-degree strokes.
Source Atle Mo
The unit cell for this seamless pattern can be had in Inkscape by selecting the rectangle and using shift+alt+i.
Source Firkin
From a drawing in 'Art Embroidery', M.S. Lockwood and E. Glaister, 1878.
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
Black paper texture, based on two different images.
Source Atle Mo
Based from Design Kindle
To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin