Seamless pattern the tile for which can be had by using shift-alt-I on the selected rectangle in Inkscape.
Source Firkin
Prismatic Abstract Geometric Background 3
Source GDJ
This is so subtle you need to bring your magnifier!
Source Carlos Valdez
Looks like an old wall. I guess that’s it then?
Source Viahorizon
From a drawing in 'Storia del Palazzo Vecchio in Firenze', Aurelio Gotti, 1889.
Source Firkin
Prismatic Geometric Tessellation Pattern 4 No Background
Source GDJ
A topographic map like this has actually been requested a few times, so here you go!
Source Sam Feyaerts
Prismatic Polyskelion Pattern No Background
Source GDJ
From a drawing in 'A Rolling Stone. A tale of wrongs and revenge', John Hartley, 1878.
Source Firkin
Prismatic Curved Diamond Pattern No Background
Source GDJ
Derived from an image that was uploaded to Pixabay by starchim01
Source Firkin
A free seamless background pattern for use on websites.
Source V. Hartikainen
8 by 8 pixels, and just what the title says.
Source pixilated
A white version of the very popular linen pattern.
Source Ant Ekşiler
ZeroCC tileable mossy (lichen) stone texture, edited from pixabay. CC0
Source Sojan Janso
Seamless , tileable CC-0 texture. Created by my own, feel free to use wherever you want!
Source Linolafett
Remixed from a drawing in 'Incidents on a Journey through Nubia to Darfoor', F. Ensor, 1891.
Source Firkin
Zero CC tileable seed texture, edited by me to be seamless from a Pixabay image. CC0
Source Sojan Janso
The starting point for this was a texture drawn with the 'Radial Colors' plug-in in Paint.net.
Source Firkin
Might not be super subtle, but quite original in its form.
Source Alex Smith
Seamless Light Background Texture.
Source V. Hartikainen
Prismatic Groovy Concentric Background 2
Source GDJ