This one is so simple, yet so good. And you know it. Has to be in the collection.
Source Gluszczenko
CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
A free seamless background with pink spots.
Source V. Hartikainen
The image is a design of blue glass.How about using it as background image?
Source Yamachem
Colourful background achieved with gradient fills.
Source Firkin
Might not be super subtle, but quite original in its form.
Source Alex Smith
Number 5 in a series of 5 beautiful patterns. Can be found in colors on the submitter’s website.
Source Janos Koos
Seamless tile drawn in Paint.net and vectorised in Vector Magic.
Source Firkin
Here I have tried to create something that would look like maple wood. Not sure how well it's turned out, but at least it looks like wood.
Source V. Hartikainen
From a drawing in 'Heroes of North African Discovery', Nancy Meugens, 1894.
Source Firkin
Remixed from an image that was uploaded to Pixabay by Pixeline
Source Firkin
Prismatic 3D Isometric Tessellation Pattern 6
Source GDJ
To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Greyscale version of a pattern that came out of playing with the 'slinky' plug-in for Paint.net
Source Firkin
Prismatic Groovy Concentric Background 2
Source GDJ
Remixed from a drawing in 'The Canadian horticulturist', 1892
Source Firkin
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
This was formed by distorting an image of a background on Pixabay.
Source Firkin
Psychedelic Geometric Background No Black
Source GDJ
An attempt for cleaning up the original image in a few steps.
Source Lazur URH
From a drawing in 'Kingsdene', Maria Fetherstonehaugh, 1878.
Source Firkin