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
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
You could get a bit dizzy from this one, but it might come in handy.
Source Dertig Media
A seamless pattern the unit cell for which can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
White fabric looking texture with some nice random wave features.
Source Hendrik Lammers
From a drawing in 'Studies for Stories', Jean Ingelow, 1864.
Source Firkin
Vector version of a png that was uploaded to Pixabay by pencilparker
Source Firkin
The first pattern on here using opacity. Try it on a site with a colored background, or even using mixed colors.
Source Nathan Spady
If you like it a bit trippy, this wave pattern might be for you.
Source Ian Soper
From a drawing in 'Hundert Jahre in Wort und Bild', S. Stefan, 1899.
Source Firkin
Sounds French. Some 3D square diagonals, that’s all you need to know.
Source Graphiste
Prismatic Geometric Pattern Background No Black
Source GDJ
This background image has seamless texture that resembles a surface of gray stone.
Source V. Hartikainen
It looks like a polished stone surface to me. Download it for free, as always.
Source V. Hartikainen
Nothing like a clean set of bed sheets, huh?
Source Badhon Ebrahim
Horizontal and vertical lines on a light gray background.
Source Adam Anlauf
Prismatic Curved Diamond Pattern 2 No Background
Source GDJ
This is so subtle: We’re talking 1% opacity. Get your squint on!
Source Atle Mo
To get the tile this is based on select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Background pattern originally a PNG drawn in Paint.net
Source Firkin