Nice and simple crossed lines in dark gray tones.
Source Stefan Aleksić
CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
Recreated from a pattern found in 'Az Osztrák-Magyar Monarchia irásban és képben', 1882. To get the tile this is based on select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Not so subtle. These tileable wood patterns are very useful.
Source Elemis
There are many carbon patterns, but this one is tiny.
Source Designova
Here's a dark background pattern that contains a steel grid pattern as a texture. Use it as a website background or for other purposes. It's free!
Source V. Hartikainen
Prismatic Abstract Background Design
Source GDJ
Here's a repeatable texture that resembles a light green concrete wall or something similar.
Source V. Hartikainen
A nice and simple white rotated tile pattern.
Source Another One
Prismatic Basic Pattern 2 No Background
Source GDJ
This tiled background comes in red and consists of tiles that look like gemstones. It is more for blogs or social profiles, I think.
Source V. Hartikainen
Formed by distorting a JPG from PublicDomainPictures
Source Firkin
Colourful background achieved with gradient fills.
Source Firkin
To get the tile this is based on select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Prismatic Snowflakes Pattern 3 No Background
Source GDJ
Seamless , tileable CC-0 texture. Created by my own, feel free to use wherever you want!
Source Linolafett
Drawn in Paint.net and vectorised in Vector Magic.
Source Firkin
That’s what it is, a dark dot. Or sort of carbon looking.
Source Tsvetelin Nikolov
Seamless pattern formed from a tile that can be had by selecting the rectangle in Inkscape and using shift-alt-i.
Source Firkin
Pixel by pixel, sharp and clean. Very light pattern with clear lines.
Source M.Ashok
And some more testing, this time with Seamless Studio. It’s Robots FFS!
Source Seamless Studio
Prismatic Groovy Concentric Background 3
Source GDJ
Remixed from a drawing in 'Line and form', Walter Crane, 1914.
Source Firkin
White fabric looking texture with some nice random wave features.
Source Hendrik Lammers