Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
Derived from an image that was uploaded to Pixabay by Darkmoon1968
Source Firkin
From a drawing in 'Art Embroidery', M.S. Lockwood and E. Glaister, 1878.
Source Firkin
A seamless texture of a rough concrete surface.
Source V. Hartikainen
Inspired by the B&O Play, I had to make this pattern.
Source Atle Mo
Classic vertical lines, in all its subtlety.
Source Cody L
Seamless , tileable CC-0 texture. Created by my own, feel free to use wherever you want!
Source Linolafett
Not a flat you live inside, like in the UK – but a flat piece of cardboard.
Source Appleshadow
A bit of scratched up grayness. Always good.
Source Dmitry
Luxury pattern, looking like it came right out of Paris.
Source Daniel Beaton
Prismatic Abstract Geometric Background 4 No Black
Source GDJ
Remixed from a JPG that was uploaded to Pixabay by susanlu4esm
Source Firkin
Design drawn in Paint.net, vectorised using Vector Magic and finished in Inkscape.
Source Firkin
A version without colours blended together to give a different look.
Source Firkin
Remixed from an image on Pixabay uploaded by Prawny
Source Firkin
Prismatic Snowflakes Pattern 3 No Background
Source GDJ
A free seamless texture of reptile skin colored in a dark brown color. As always, you may use it as a repeated background image in your web design works, or for any other purposes.
Source V. Hartikainen
Pattern formed from simple shapes. Black version.
Source Firkin
The square tile this is based on can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin
Tiny little fibers making a soft and sweet look.
Source Badhon Ebrahim
Light gray grunge wall with a nice texture overlay.
Source Adam Anlauf
A seamless pattern with a unit cell drawn as a bitmap in Paint.net and vectorized in Vector Magic.
Source Firkin