A seamless pattern based on a rectangular tile that can be retrieved in Inkscape by selecting the rectangle and using shift-alt-i.
Source Firkin
A free seamless background texture of "timber wall" (colored in dark brown).
Source V. Hartikainen
As far as fabric patterns goes, this is quite crisp.
Source Heliodor Jalba
Abstract Tiled Background Extended 12
Source GDJ
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
A comeback for you: the popular Escheresque, now in black.
Source Patten
Prismatic Groovy Concentric Background 2
Source GDJ
A free repetitive background with a dark concrete wall like texture. This one may be used in dark web site designs.
Source V. Hartikainen
This is so subtle: We’re talking 1% opacity. Get your squint on!
Source Atle Mo
Same as gray sand but lighter. A sandy pattern with small light dots, and some angled strokes.
Source Atle Mo
Prismatic Abstract Geometric Background derived from an image on Pixabay.
Source GDJ
A dark pattern made out of 3×3 circles and a 1px shadow. This works well as a carbon texture or background.
Source Atle Mo
Background Wall, Art Abstract, white Well & CC0 texture.
Source Ractapopulous
Remixed from an image on Pixabay, the original having been uploaded by darkmoon1968.
Source Firkin
Zero CC tileable ground (#2) cracked, crackled texture, made by me. CC0
Source Sojan Janso
Actually remixed from a pattern on Pixabay. But then noticed a very similar one on Openclipart.org uploaded by btj51q2.
Source Firkin
A seamless pattern formed from miutopia mug remixes on a tablecloth.
Source Firkin
Cubes as far as your eyes can see. You know, because they tile.
Source Jan Meeus
A seamless paper background colored in pale yellow.
Source V. Hartikainen
Prismatic Rounded Squares Grid 3 No Background
Source GDJ
From a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin