A grid of squares with green colours. Since the colours are randomly distributed it is automatically seamless.
Source Firkin
Like the name says, light and gray, with some small dots and circles.
Source Brenda Lay
Light gray version of the Binding pattern that looks a bit like fabric.
Source Newbury
An interesting dark spotted pattern at an angle.
Source Hendrik Lammers
Background pattern originally a PNG drawn in Paint.net
Source Firkin
Remixed from a drawing in 'Line and form', Walter Crane, 1914.
Source Firkin
The image is a remix of "edo pattern-samekomon".I changed the color of dots from black to white and added BG in light-yellow.
Source Yamachem
A monochrome pattern from a tile that can be had by selecting the rectangle in Inkscaope and using shift+alt+i
Source Firkin
Looks like a technical drawing board: small squares forming a nice grid.
Source We Are Pixel8
To get the tile this is based on select the rectangle in Inkscape and use shift-alt-i
Source Firkin
A dark gray, sandy pattern with small light dots, and some angled strokes.
Source Atle Mo
A fun-looking elastoplast/band-aid pattern. A hint of orange tone in this one.
Source Josh Green
Design drawn in Paint.net, vectorised using Vector Magic and finished in Inkscape.
Source Firkin
It’s big, it’s gradient—and it’s square.
Source Brankic1979
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
You guessed it – looks a bit like cloth.
Source Peax Webdesign
Prismatic Basic Pattern 2 No Background
Source GDJ
Prismatic Abstract Geometric Background 2 No Black
Source GDJ
CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
An emulated “transparent” background pattern, like that of all kinds of computer graphics software.
Source AdamStanislav
Drawn in Paint.net and vectorised in Vector Magic.
Source Firkin
Black brick wall pattern. Brick your site up!
Source Alex Parker