A seamless pattern the starting point for which was a 'colour modulo' texture in Paint.net.
Source Firkin
A seamless pattern formed from a square tile. To get the tile, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Washi (和紙?) is a type of paper made in Japan. Here’s the pattern for you!
Source Carolynne
Tiny little fibers making a soft and sweet look.
Source Badhon Ebrahim
A seamless pattern formed from a tile made from ornament 22. To get the tile, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Pixel by pixel, sharp and clean. Very light pattern with clear lines.
Source M.Ashok
From a drawing in 'At home', J. Sowerby, J. Crane and T. Frederick, 1881.
Source Firkin
Vector version of a JPG that was uploaded to Pixabay by theasad121
Source Firkin
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
The green fibers pattern will work very well in grayscale as well.
Source Matteo Di Capua
Just like your old suit, all striped and smooth.
Source Alex Berkowitz
This is a semi-dark pattern, sort of linen-y.
Source Sagive SEO
Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
Sharp but soft triangles in light shades of gray.
Source Pixeden
Derived from an image that was uploaded to Pixabay by darkmoon1968
Source Firkin
Abstract Arbitrary Geometric Background derived from an image on Pixabay.
Source GDJ
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
This background image has seamless texture that resembles a surface of gray stone.
Source V. Hartikainen
Zero CC tillable hard cover red book with X shape marks. Scanned and made by me.
Source Sojan Janso
The image depicts a seamless pattern made using a bird's face.
Source Yamachem
Light gray pattern with an almost wall tile-like appearance.
Source Markus Tinner