As simple and subtle as it gets. But sometimes that’s just what you want.
Source Designova
Prismatic Triangular Seamless Pattern III With Background
Source GDJ
This was formed by distorting an image of a background on Pixabay.
Source Firkin
The green fibers pattern will work very well in grayscale as well.
Source Matteo Di Capua
Super simple but very nice indeed. Gray with vertical stripes.
Source Merrin Macleod
Prismatic Groovy Concentric Background No Black
Source GDJ
CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
A seamless background drawn in Paint.net and vectorised with Vector Magic. The starting point was a photograph of drinking straws from Pixabay.
Source Firkin
Derived from an image that was uploaded to Pixabay by mdmelo.
Source Firkin
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
A pattern derived from repeating unit cells each derived from part of a fractal rendering in paint.net.
Source Firkin
The classic notebook paper with horizontal stripes.
Source Are Sundnes
Medium gray fabric pattern with 45-degree lines going across.
Source Atle Mo
Prismatic Hypnotic Pattern No Background
Source GDJ
From a drawing in 'At home', J. Sowerby, J. Crane and T. Frederick, 1881.
Source Firkin
Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
Alternative colour scheme to the original.
Source Firkin
Prismatic Polka Dots Mark II No Background
Source GDJ
Zero CC tileable bark texture, photographed and made by me. CC0
Source Sojan Janso
Same as gray sand but lighter. A sandy pattern with small light dots, and some angled strokes.
Source Atle Mo
A bit like smudged paint or some sort of steel, here is scribble light.
Source Tegan Male
Dark and hard, just the way we like it. Embossed triangles makes a nice pattern.
Source Ivan Ginev
From a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
I took the liberty of using Dmitry’s pattern and made a version without perforation.
Source Atle Mo