From an image on opengameart.org shared by rubberduck.
Source Firkin
Prismatic Hypnotic Pattern No Background
Source GDJ
From a drawing in 'In an Enchanted Island', William Mallock, 1892.
Source Firkin
Prismatic Abstract Geometric Background 4 No Black
Source GDJ
Prismatic Abstract Background Design
Source GDJ
CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
It has waves, so make sure you don’t get sea sickness.
Source CoolPatterns
Adapted heavily from a JPG that was uploaded to Pixabay by Viscious-Speed.
Source Firkin
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
A series of 5 patterns. That’s what the P stands for, if you didn’t guess it.
Source Dima Shiper
Remixed from a drawing in 'The Canadian horticulturist', 1892
Source Firkin
I took the liberty of using Dmitry’s pattern and made a version without perforation.
Source Atle Mo
A repeating background with wood/straw like texture.
Source V. Hartikainen
Zero CC bark from fur tree tileable texture, photographed and made by me. CC0
Source Sojan Janso
Prismatic Rounded Squares Grid 4 No Background
Source GDJ
Formed from a tile based on a drawing from 'Viaggi d'un artista nell'America Meridionale', Guido Boggiani, 1895.
Source Firkin
A monochrome pattern from a tile that can be had by selecting the rectangle in Inkscaope and using shift+alt+i
Source Firkin
Prismatic Abstract Line Art Pattern Background 2
Source GDJ
A seamless pattern based on a square tile that can be retrieved in Inkscape by selecting the rectangle and using shift-alt-i.
Source Firkin
From a drawing in 'Navigations de Alouys de Cademoste.-La Navigation du Capitaine Pierre Sintre', Alvise da ca da Mosto, 1895.
Source Firkin