A seamlessly tileable pink background texture.
Source V. Hartikainen
A grid of squares with green colours. Since the colours are randomly distributed it is automatically seamless.
Source Firkin
From a drawing in 'Cassell's Library of English Literature', Henry Morley, 1883.
Source Firkin
This one is super crisp at 2X. Lined paper with some dust and scratches.
Source HQvectors
Heavily remixed from a drawing that was uploaded to Pixabay by ractapopulous
Source Firkin
Retro Circles Background 7 No Black
Source GDJ
Dark squares with some virus-looking dots in the grid.
Source Hugo Loning
A seamless pattern formed from a modified version of rwwgub's tile. To get the tile, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Got some felt in my mailbox today, so I scanned it for you to use.
Source Atle Mo
If you want png files of this u can download them here : viscious-speed.deviantart.com/gallery/27635117
Source Viscious-Speed
Prismatic Polka Dots Mark II No Background
Source GDJ
A re-make of the Gradient Squares pattern.
Source Dimitar Karaytchev
Looks like an old wall. I guess that’s it then?
Source Viahorizon
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
Alternative colour scheme. Not a pattern for fabrics, but one produced from a jpg of a stack of fabric items that was posted on Pixabay. The tile that this is based on can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Prismatic Curved Diamond Pattern 6 No Background
Source GDJ
Might not be super subtle, but quite original in its form.
Source Alex Smith
Nice and simple crossed lines in dark gray tones.
Source Stefan Aleksić
This is a grid, only it’s noisy. You know. Reminds you of those printed grids you draw on.
Source Vectorpile
You may use it as is, or modify it as you like.
Source V. Hartikainen
Abstract Stars Geometric Pattern Prismatic No Background
Source GDJ
Imagine you zoomed in 1000X on some fabric. But then it turned out to be a skeleton!
Source Angelica
A seamless pattern with a unit cell drawn as a bitmap in Paint.net and vectorized in Vector Magic.
Source Firkin
Embossed lines and squares with subtle highlights.
Source Alex Parker
Prismatic Groovy Concentric Background 2
Source GDJ