Here's a subtle marble-like background for use on websites.
Source V. Hartikainen
Remixed from a design seen on Pixabay. The basic tile can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Remixed from a vector adapted from a jpg on Pixabay. The tile this is constructed from can be had by selecting the rectangle in Inkscape and using shift-alt-i.
Source Firkin
Prismatic Groovy Concentric Background 2
Source GDJ
The image depicts a seamless pattern of Japanese Edo pattern called "kikkou-matsu" or "亀甲松" meaning " tortoiseshell-pinetree".The real pinetree is like this: https://jp.pinterest.com/pin/500744052301065077/
Source Yamachem
Continuing the geometric trend, here is one more.
Source Mike Warner
Free tiled background with colorful stripes and white splatter.
Source V. Hartikainen
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
Little x’es, noise and all the stuff you like. Dark like a Monday, with a hint of blue.
Source Tom McArdle
Fake or not, it’s quite luxurious.
Source Factorio.us Collective
If you want png files of this u can download them here : viscious-speed.deviantart.com/gallery/27635117
Source Viscious-Speed
Dark, lines, noise, tactile. You get the drift.
Source Anatoli Nicolae
Background Wall, Art Abstract, Blue Well & CC0 texture.
Source Ractapopulous
To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
There are many carbon patterns, but this one is tiny.
Source Designova
ZeroCC tileable mossy (lichen) stone texture, edited from pixabay. CC0
Source Sojan Janso
Pattern that came out of playing with the 'slinky' plug-in for Paint.net
Source Firkin
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
It looks like a polished stone surface to me. Download it for free, as always.
Source V. Hartikainen
You may use it as is, or modify it as you like.
Source V. Hartikainen
Three shades of gray makes this pattern look like a small carbon fiber surface. Great readability even for small fonts.
Source Atle Mo
This is a seamless pattern which is derived from a flower petal image.
Source Yamachem
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
Design drawn in Paint.net, vectorised using Vector Magic and finished in Inkscape.
Source Firkin
A free seamless background texture of "timber wall" (colored in dark brown).
Source V. Hartikainen
The starting point for this was drawn on the web site steamcoded.org/PolyskelionMaker.svg
Source Firkin