Seamless pattern formed from a tile that can be extracted by selecting the rectangle in Inkscape and using shift-alt-i.
Source Firkin
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
A light gray fabric pattern with faded vertical stripes.
Source V. Hartikainen
Floral patterns will never go out of style, so enjoy this one.
Source Lasma
Colour version of the original pattern.
Source Firkin
Very dark pattern with some noise and 45-degree lines.
Source Stefan Aleksić
Colour version of a pattern that came out of playing with the 'light rays' plug-in for Paint.net
Source Firkin
A topographic map like this has actually been requested a few times, so here you go!
Source Sam Feyaerts
A pale olive green background with a seamless texture.
Source V. Hartikainen
Utilising a bird from s-light and some flowers from Almeidah. To get the unit tile, select the rectangle in Inkscape and use shift-alt-i.
Source Firkin
Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
Zero CC tileable ground (#2) cracked, crackled texture, made by me. CC0
Source Sojan Janso
Small gradient crosses inside 45-degree boxes, or bigger crosses if you will.
Source Wassim
Derived from a drawing in 'The Murmur of the Shells', Samuel Cowen, 1879.
Source Firkin
This pattern comes in orange, and it looks as if it is "made of glass".
Source V. Hartikainen
Remixed from a design seen on Pixabay. The basic tile can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
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
This was formed by distorting an image of a background on Pixabay.
Source Firkin
Some rectangles, a bit of dust and grunge, plus a hint of concrete.
Source Atle Mo
8 by 8 pixels, and just what the title says.
Source pixilated
From a drawing in 'Line and form', Walter Crane, 1914.
Source Firkin