People seem to enjoy dark patterns, so here is one with some circles.
Source Atle Mo
The image depicts the Japanese Edo pattern called "seigaiha" or "青海波" meaning "blue -sea- wave".I hope it's suitable for the summer season.
Source Yamachem
A version without colours blended together to give a different look.
Source Firkin
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
This one takes you back to math class. Classic mathematic board underlay.
Source Josh Green
A seamless pattern the starting point for which was a 'rainbow twist' texture in Paint.net.
Source Firkin
I have no idea what J Boo means by this name, but hey – it’s hot.
Source j Boo
From a drawing in 'Sun Pictures of the Norfolk Broads', Ernest Suffling, 1892.
Source Firkin
Fake or not, it’s quite luxurious.
Source Factorio.us Collective
A classic dark tile for a bit of vintage darkness.
Source Listvetra
A monochrome pattern from a tile that can be had by selecting the rectangle in Inkscaope and using shift+alt+i
Source Firkin
Drawn in Paint.net and vectorised in Vector Magic.
Source Firkin
Prismatic Abstract Background Design No Black
Source GDJ
From a drawing in 'At home', J. Sowerby, J. Crane and T. Frederick, 1881.
Source Firkin
White little knobs, coming in at 10x10px. Sweet!
Source Amos
Turn your site into a dragon with this great scale pattern.
Source Alex Parker
The classic subtle pattern. Sort of wall/brick looking. Or moon-looking?
Source Joel Klein
Seamless pattern formed from a tile that can be had by selecting the rectangle in Inkscape and using shift-alt-i.
Source Firkin
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
This was formed by distorting an image of a background on Pixabay.
Source Firkin
Remixed from a drawing in 'Line and form', Walter Crane, 1914.
Source Firkin
This is indeed a bit strange, but here’s to the crazy ones!
Source Christopher Buecheler
Light honeycomb pattern made up of the classic hexagon shape.
Source Federica Pelzel
If you want png files of this u can download them here : viscious-speed.deviantart.com/gallery/27635117
Source Viscious-Speed
CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
An alternative colour scheme for the original background.
Source Firkin
Produced using the clouds, flames and glass blocks plug-ins in Paint.net and the resulting .PNG vectorised with Vector Magic.
Source Firkin