An abstract texture of water. It's not perfect, but will do. You may download if you like it.
Source V. Hartikainen
Here's a subtle marble-like background for use on websites.
Source V. Hartikainen
Prismatic Groovy Concentric Background 5 No Black
Source GDJ
Prismatic Geometric Tessellation Pattern No Background
Source GDJ
A seamless texture of worn out "cardboard".
Source V. Hartikainen
From a drawing in 'Codogno e il suo territorio nella cronaca e nella storia'', Gio and Giarella Cairo, 1897.
Source Firkin
Remixed from a drawing in 'The Canadian horticulturist', 1892
Source Firkin
From a drawing in 'Art Embroidery', M.S. Lockwood and E. Glaister, 1878.
Source Firkin
I have no idea what J Boo means by this name, but hey – it’s hot.
Source j Boo
Prismatic Abstract Geometric Background 4
Source GDJ
Remixed from an image that was uploaded to Pixabay by Pixeline
Source Firkin
From a drawing in 'Cassell's Library of English Literature', Henry Morley, 1883.
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
Some more diagonal lines and noise, because you know you want it.
Source Atle Mo
Background pattern originally a PNG drawn in Paint.net
Source Firkin
The unit cell for this seamless pattern can be had in Inkscape by selecting the rectangle and using shift+alt+i.
Source Firkin
Fabric-ish patterns are close to my heart. French Stucco to the rescue.
Source Christopher Buecheler
Hexagonal dark 3D pattern. What more can you ask for?
Source Norbert Levajsics
To get the tile this is based on select the rectangle in Inkscape and use shift+alt+i.
Source Firkin