CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
The tile this is based on can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin
This beige background pattern resembles a concrete wall with engravings or something similar to it.
Source V. Hartikainen
Background formed from the original with an emboss effect.
Source Firkin
Prismatic Groovy Concentric Background 4
Source GDJ
Based on an image that was uploaded to Pixabay by devanath
Source Firkin
From a drawing in 'Two Women in the Klondike', Mary Hitchcock, 1899.
Source Firkin
Nice and simple crossed lines in dark gray tones.
Source Stefan Aleksić
A topographic map like this has actually been requested a few times, so here you go!
Source Sam Feyaerts
Prismatic Curved Diamond Pattern 4 No Background
Source GDJ
Continuing the geometric trend, here is one more.
Source Mike Warner
This pack of filters can help you adding a blocky overlay to objects. May come handy at drawing blocks of stone.
Source Lazur URH
Dark, crisp and subtle. Tiny black lines on top of some noise.
Source Wilmotte Bastien
Seamless Light Background Texture.
Source V. Hartikainen
Old China with a modern twist, take two.
Source Adam Charlts
Alternative colour scheme. 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
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
Just to prove my point, here is a slightly modified dark version.
Source Atle Mo
This one has rusty dark brown texture.
Source V. Hartikainen
Remixed from a drawing in 'Line and form", Walter Crane, 1914.
Source Firkin
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
The green fibers pattern will work very well in grayscale as well.
Source Matteo Di Capua
Prismatic Snowflakes Pattern No Background
Source GDJ
To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin