The green fibers pattern will work very well in grayscale as well.
Source Matteo Di Capua
A playful triangle pattern with different shades of gray.
Source Dimitrie Hoekstra
More Japanese-inspired patterns, Gold Scales this time.
Source Josh Green
Remixed from a JPG that was uploaded to Pixabay by Osckar
Source Firkin
A seamless pattern formed from a modified version of rwwgub's tile. To get the tile, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Dark, lines, noise, tactile. You get the drift.
Source Anatoli Nicolae
Greyscale version of a pattern that came out of playing with the 'light rays' plug-in for Paint.net
Source Firkin
Might not be super subtle, but quite original in its form.
Source Alex Smith
A beautiful dark padded pattern, like an old classic sofa.
Source Chris Baldie
Dark pattern with some nice diagonal stitched lines crossing over.
Source Ashton
A seamless striped fabric-like texture colored in a dark reddish brown color.
Source V. Hartikainen
On a large canvas you can see it tiling, but used on smaller areas, it’s beautiful.
Source Paul Phönixweiß
A seamless green background texture. The image is distributed under a Creative Commons License (like all of the images here).
Source V. Hartikainen
Don’t look at this one too long if you’re high on something.
Source Luuk van Baars
A light gray fabric pattern with faded vertical stripes.
Source V. Hartikainen
Dark, crisp and subtle. Tiny black lines on top of some noise.
Source Wilmotte Bastien
Sometimes you just need the simplest thing.
Source Fabricio
This is a remix of "blue wave-seigaiha".I hope this subtle color version of Seigaiha would be suitable for background .
Source Yamachem
Background formed from the iconic plastic construction bricks that gave me endless hours of fun when I was a lad.
Source Firkin