Because I love dark patterns, here is Brushed Alum in a dark coating.
Source Tim Ward
This one resembles a black concrete wall when is tiled. It should look great, at least with dark website themes.
Source V. Hartikainen
This one has rusty dark brown texture.
Source V. Hartikainen
Prismatic Snowflakes Pattern No Background
Source GDJ
ZeroCC tileable stone texture, edited from pixabay, CC0
Source Sojan Janso
A playful triangle pattern with different shades of gray.
Source Dimitrie Hoekstra
To get the tile this is based on select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Prismatic Groovy Concentric Background 3 No Black
Source GDJ
Drawn in Paint.net and vectorised in Vector Magic
Source Firkin
Number five from the same submitter, makes my job easy.
Source Dima Shiper
The tile for this is based on a repeating unit close to a design on Pixabay. It can be had by selecting the rectangle in Inkscape and using shift-alt-i.
Source Firkin
Remixed from a drawing in 'The Canadian horticulturist', 1892
Source Firkin
A slightly more textured pattern, medium gray. A bit like a potato sack?
Source Bilal Ketab
Retro Circles Background 5 No Black
Source GDJ
From a drawing in 'Real Sailor-Songs', John Ashton, 1891.
Source Firkin
A seamless pattern the starting point for which was a 'colour modulo' texture in Paint.net.
Source Firkin
A dark metallic background with a pattern of stamped dots. Here's a dark "metallic" background pattern for you.
Source V. Hartikainen
Remixed from a drawing in 'The March of Loyalty', Letitia MacClintock, 1884.
Source Firkin
A seamless pattern formed from a rectangular tile. The tile can be retrieved by selecting the rectangle in Inkscape and using shift-alt-i.
Source Firkin
A seamless web texture of "green stone".
Source V. Hartikainen
Nice and simple crossed lines in dark gray tones.
Source Stefan Aleksić
Tiny, tiny 3D cubes. Reminds me of the good old pattern from k10k.
Source Etienne Rallion
Remix from a drawing in 'Ostatnie chwile powstania styczniowego', Zygmunt Sulima, 1887.
Source Firkin
The tile this is formed from can be retrieved in Inkscape by selecting the rectangle and using shift+alt+i
Source Firkin
Vector version of a png that was uploaded to Pixabay by pencilparker
Source Firkin
The image depicts a seamless pattern of a Japanese family crest called "chidori" in Japanese .A chidori in Japanese means a plover in English.
Source Yamachem