Did some testing with Repper Pro tonight, and this gray mid-tone pattern came out.
Source Atle Mo
A grid of squares with green colours. Since the colours are randomly distributed it is automatically seamless.
Source Firkin
Black & white version of a pattern that came out of playing with the 'light rays' plug-in for Paint.net
Source Firkin
Thin lines, noise and texture creates this crisp dark denim pattern.
Source Marco Slooten
A very dark spotted twinkle pattern for your twinkle needs.
Source Badhon Ebrahim
A seamlessly repeating background pattern of wood. The image is procedurally generated, and, I think, it's turned out quite well.
Source V. Hartikainen
Based on several public domain drawings on Wikimedia Commons. This was formed from a rectangular tile. The tile can be accessed in Inkscape by selecting the rectangle and using shift-alt-i
Source Firkin
It looks like a polished stone surface to me. Download it for free, as always.
Source V. Hartikainen
From a drawing in 'Art Embroidery', M.S. Lockwood and E. Glaister, 1878.
Source Firkin
A hint of orange color, and some crossed and embossed lines.
Source Adam Anlauf
The following repeating website background is colored in a blue gray color and resembles a concrete wall or something similar to it.
Source V. Hartikainen
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
A smooth mid-tone gray, or low contrast if you will, linen pattern.
Source Jordan Pittman
A seamless marble-like texture colored in light blue.
Source V. Hartikainen
A nice and simple white rotated tile pattern.
Source Another One
As simple and subtle as it gets. But sometimes that’s just what you want.
Source Designova
From a drawing in 'Art Embroidery', M.S. Lockwood and E. Glaister, 1878.
Source Firkin
Background formed from the iconic plastic construction bricks that gave me endless hours of fun when I was a lad.
Source Firkin
Sort of like the Photoshop transparent background, but better!
Source Alex Parker
This one needs to be used in small areas; you can see it repeat.
Source Luca
This is the remix of an OCAL clipart called "Rain on Window" uploaded by "pagarmidna".Thanks.This is a seamless pattern of raindrops.
Source Yamachem
Used correctly, this could be nice. Used in a bad way, all hell will break loose.
Source Atle Mo
Derived from an image that was uploaded to Pixabay by mdmelo.
Source Firkin
Dead simple but beautiful horizontal line pattern.
Source Fabian Schultz
Prismatic Groovy Concentric Background 4 No Black
Source GDJ