Repeating squares overlapping.
Source Pete Fecteau
Used a cherry by doctormo to make this seamless pattern
Source Firkin
Thin lines, noise and texture creates this crisp dark denim pattern.
Source Marco Slooten
Prismatic Abstract Background Design
Source GDJ
A black tile-able background with paper-like texture.
Source V. Hartikainen
A topographic map like this has actually been requested a few times, so here you go!
Source Sam Feyaerts
From a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Dark wooden pattern, given the subtle treatment. based on texture from Cloaks. https://cloaks.deviantart.com
Source Atle Mo
To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Sometimes you just need the simplest thing.
Source Fabricio
A grid of squares with green colours. Since the colours are randomly distributed it is automatically seamless.
Source Firkin
Background pattern originally a PNG drawn in Paint.net
Source Firkin
Prismatic Abstract Line Art Pattern Background 2
Source GDJ
This one resembles a black concrete wall when is tiled. It should look great, at least with dark website themes.
Source V. Hartikainen
Remixed from a drawing that was uploaded to Pixabay by ractapopulous
Source Firkin
A repeating background of beige paper with vintage look. Repeats to infinity, as usual.
Source V. Hartikainen
Zero CC tileable hard cover cells book texture, 4k, scanned and made by me. CC0
Source Sojan Janso
Dark, crisp and subtle. Tiny black lines on top of some noise.
Source Wilmotte Bastien
Not a flat you live inside, like in the UK – but a flat piece of cardboard.
Source Appleshadow
Seamless , tileable CC-0 texture. Created by my own, feel free to use wherever you want!
Source Linolafett
It’s an egg, in the form of a pattern. This really is 2012.
Source Paul Phönixweiß
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
Pattern Background, Texture, Photoshop Structure style CC0 texture.
Source Darkmoon1968