I guess this is inspired by the city of Ravenna in Italy and its stone walls.
Source Sentel
From a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
A repeating gloomy background image. This one consists of a pattern of black chains layered on top of a dark textured background.
Source V. Hartikainen
ZeroCC tileable wood boards texture, photographed and made by me. CC0
Source Sojan Janso
Prismatic Floral Background No Black
Source GDJ
Kaleidoscope Prismatic Abstract No Background
Source GDJ
A version without colours blended together to give a different look.
Source Firkin
Looks like a technical drawing board: small squares forming a nice grid.
Source We Are Pixel8
From a drawing in 'Art Embroidery', M.S. Lockwood and E. Glaister, 1878.
Source Firkin
Retro Circles Background 5 No Black
Source GDJ
Might not be super subtle, but quite original in its form.
Source Alex Smith
The image a seamless pattern derived from a weed which I can't identify.The original weed image is from here:https://jp.pinterest.com/pin/500744052301423641/
Source Yamachem
Prismatic Isometric Cube Extra Pattern No Background
Source GDJ
You don’t see many mid-tone patterns here, but this one is nice.
Source Joel Klein
A dark metal plate with an embossed grid pattern and a bit of rust. Here's a dark metal plate texture for use as a tiled background on web pages.
Source V. Hartikainen
Very simple, very blu(e). Subtle and nice.
Source Seb Jachec
From a drawing in 'Cowdray: the history of a great English House', Julia Roundell, 1884.
Source Firkin
Bigger is better, right? So here you have some large carbon fiber.
Source Factorio.us Collective
If you like it a bit trippy, this wave pattern might be for you.
Source Ian Soper
Zero CC tileable pine bark texture, photographed and made by me. CC0
Source Sojan Janso
A free web background image with a seamless concrete-like texture and an Indian-red color.
Source V. Hartikainen
A seamless background texture of old cardboard.
Source V. Hartikainen
A chequerboard pattern with a fruit theme. The fruits are from a posting by inkscapeforum.it.
Source Firkin