A seamless green background texture. The image is distributed under a Creative Commons License (like all of the images here).
Source V. Hartikainen
Prismatic Groovy Concentric Background 5
Source GDJ
Prismatic Basic Pattern 2 No Background
Source GDJ
This is so subtle I hope you can see it! Tweak at will.
Source Alexandre Naud
A black tile-able background with paper-like texture.
Source V. Hartikainen
Remixed from a drawing that was uploaded to Pixabay by ractapopulous
Source Firkin
Prismatic Abstract Background Design
Source GDJ
Remixed from a drawing in 'Chambéry à la fin du XIVe siècle', Timoleon Chapperon, 1863.
Source Firkin
We have some linen patterns here, but none that are stressed. Until now.
Source Jordan Pittman
A pattern derived from repeating unit cells each derived from part of a fractal rendering in paint.net.
Source Firkin
Inspired by a pattern found in 'Sun Pictures of the Norfolk Broads', Ernest Suffling, 1892.
Source Firkin
Seamless pattern formed from a tile that can be had by selecting the rectangle in Inkscape and using shift-alt-i.
Source Firkin
One more sharp little tile for you. Subtle circles this time.
Source Blunia
Remixed from a design seen in 'Burghley. The Life of William Cecil', William Charlton, 1857. The tile this is based on can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Zero CC tileable bark texture, photographed and made by me. CC0
Source Sojan Janso
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
From a drawing in 'Less Black than we're painted', James Payn, 1884.
Source Firkin
A topographic map like this has actually been requested a few times, so here you go!
Source Sam Feyaerts
An emulated “transparent” background pattern, like that of all kinds of computer graphics software.
Source AdamStanislav
Not a flat you live inside, like in the UK – but a flat piece of cardboard.
Source Appleshadow