This is so subtle you need to bring your magnifier!
Source Carlos Valdez
Derived from an image that was uploaded to Pixabay by darkmoon1968
Source Firkin
From a drawing in 'Uit de geschiedenis der Heilige Stede te Amsterdam', Yohannes Sterck, 1898.
Source Firkin
A seamless background drawn in Paint.net and vectorised with Vector Magic. The starting point was a photograph of drinking straws from Pixabay.
Source Firkin
The unit cell for this seamless pattern can be had in Inkscape by selecting the rectangle and using shift+alt+i.
Source Firkin
Subtle scratches on a light gray background.
Source Andrey Ovcharov
A dark metal plate with an embossed grid pattern and a bit of rust. Here's a dark metal plate texture for use as a tiled background on web pages.
Source V. Hartikainen
A dark gray, sandy pattern with small light dots, and some angled strokes.
Source Atle Mo
CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
Don’t look at this one too long if you’re high on something.
Source Luuk van Baars
Sharp pixel pattern looking like some sort of fabric.
Source Dmitry
An alternative colour scheme to the original seamless pattern.
Source Firkin
This is a remix of "blue wave-seigaiha".I hope this subtle color version of Seigaiha would be suitable for background .
Source Yamachem
The name alone is awesome, but so is this sweet dark pattern.
Source Federica Pelzel
From a drawing in 'Picturesque New Guinea', J Lindt, 1887.
Source Firkin
A seamless pattern formed from a square tile. The tile can be retrieved by selecting the rectangle in Inkscape and using shift-alt-i.
Source Firkin
From a drawing in 'Line and form', Walter Crane, 1914.
Source Firkin
An abstract pale yellow paper-like background with stains colored in yellow and green.
Source V. Hartikainen
Have you wondered about how it feels to be buried alive? Here is the pattern for it.
Source Hendrik Lammers
Background Wall, Art Abstract, Block Well & CC0 texture.
Source Ractapopulous
A hint of orange color, and some crossed and embossed lines.
Source Adam Anlauf
A seamless web background with texture of aged grid paper.
Source V. Hartikainen