The green fibers pattern will work very well in grayscale as well.
Source Matteo Di Capua
Not the most creative name, but it’s a good all-purpose light background.
Source Dmitry
Dark Tile-able Grunge Texture. I think this texture can be classified as grunge. It's free and seamless, as always.
Source V. Hartikainen
Kaleidoscope Prismatic Abstract No Background
Source GDJ
This was formed by distorting an image of a background on Pixabay.
Source Firkin
The act or state of corrugating or of being corrugated, a wrinkle; fold; furrow; ridge.
Source Anna Litvinuk
Dark, square, clean and tidy. What more can you ask for?
Source Jaromír Kavan
Remixed from a design on Pixabay. To get the tile this is based on select the rectangle in Inkscape and use shift-alt-i.
Source Firkin
Remixed from a PNG that was uploaded to Pixabay by k_jprather
Source Firkin
Zero CC bark from fur tree tileable texture, photographed and made by me. CC0
Source Sojan Janso
Remixed from a drawing in 'Maidenhood; or, the Verge of the Stream', Laura Jewry, 1876.
Source Firkin
From a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Number 4 in a series of 5 beautiful patterns. Can be found in colors on the submitter’s website.
Source Janos Koos
Prismatic Geometric Pattern Background 2 No Black
Source GDJ
A playful triangle pattern with different shades of gray.
Source Dimitrie Hoekstra
A repeatable image with dark background and metal grid pattern.
Source V. Hartikainen
From a drawing in 'Artists and Arabs', Henry Blackburn, 1868
Source Firkin
Prismatic Groovy Concentric Background 3
Source GDJ
A bit simplified version. Although it could be edited out to be simpler. Anyway, this time the tiling is converted to a pattern fill -which is using clipping for the tile's edges.
Source Lazur URH
A free repetitive background with a dark concrete wall like texture. This one may be used in dark web site designs.
Source V. Hartikainen
Remixed from a raster on Pixabay that was uploaded by ArtsyBee.
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
Carbon fiber is never out of fashion, so here is one more style for you.
Source Alfred Lee