Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
I’m starting to think I have a concrete wall fetish.
Source Atle Mo
The file was named striped lens, but hey – Translucent Fibres works too.
Source Angelica
Prismatic Geometric Pattern Variation 2 With Background
Source GDJ
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
From a design found in 'History of the Virginia Company of London; with letters to and from the first Colony, never before printed', Edward Neill, 1869.
Source Firkin
Inspired by a design found in 'Konstantinápolyi emlékeim', Miklos Chriszto, 1893.
Source Firkin
The name is totally random, but hey, it sounds good.
Source Atle Mo
From a drawing in 'Resa i Afrika, genom Angola, Ovampo och Damaraland', P. Moller, 1899.
Source Firkin
A grid of squares with green colours. Since the colours are randomly distributed it is automatically seamless.
Source Firkin
All good things come in threes, so I give you the third in my little concrete wall series.
Source Atle Mo
From a drawing in 'Two Women in the Klondike', Mary Hitchcock, 1899.
Source Firkin
If you want png files of this u can download them here : viscious-speed.deviantart.com/gallery/27635117
Source Viscious-Speed
Imagine you zoomed in 1000X on some fabric. But then it turned out to be a skeleton!
Source Angelica
To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
It’s like Shine Dotted’s sister, only rotated 45 degrees.
Source mediumidee
I know there is one here already, but this is sexy!
Source Gjermund Gustavsen
Here's a new paper-like background for free use on personal and commercial projects (this applies to all background patterns here).
Source V. Hartikainen
A seamless pattern based on a square tile that can be retrieved in Inkscape by selecting the rectangle and using shift-alt-i.
Source Firkin