Here's a subtle marble-like background for use on websites.
Source V. Hartikainen
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
I guess this one is inspired by an office. A dark office.
Source Andrés Rigo.
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
Abstract Arbitrary Geometric Background derived from an image on Pixabay.
Source GDJ
Used a cherry by doctormo to make this seamless pattern
Source Firkin
No idea what Nistri means, but it’s a crisp little pattern nonetheless.
Source Markus Reiter
Utilising some flowers from Almeidah. To get the unit tile, select the rectangle in Inkscape and use shift-alt-i.
Source Firkin
A huge one at 800x600px. Made from a photo I took going home after work.
Source Atle Mo
Prismatic Geometric Pattern Background
Source GDJ
You don’t see many mid-tone patterns here, but this one is nice.
Source Joel Klein
CC0 remixed from a drawing. Walter Crane, 1914, Firkin.
Source SliverKnight
A lovely light gray pattern with stripes and a dash of noise.
Source V. Hartikainen
The rectangular tile this is based on can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin
Derived from an image that was uploaded to Pixabay by Darkmoon1968
Source Firkin
Classy golf-pants pattern, or crossed stripes if you will.
Source Will Monson
Background Wall, Art Abstract, Star Well & CC0 texture.
Source Ractapopulous
A seamless chequerboard pattern formed from a tile that can be had in Inkscape by selecting the rectangle and using shift+alt+i. Alternative colour scheme.
Source Firkin
Prismatic Groovy Concentric Background 3
Source GDJ
Super dark, crisp and detailed. And a Kill Bill reference.
Source Badhon Ebrahim
This ons is quite old school looking. Retro, even. I like it.
Source Arno Declercq
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
A comeback for you: the popular Escheresque, now in black.
Source Patten