A simple example on using clones. You can generate a nice base for a pattern fill quickly with it.
Source Lazur URH
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
Brushed aluminum, in a bright gray version. Lovely 2X as well.
Source Andre Schouten
Crossing lines with a subtle emboss effect on a dark background.
Source Stefan Aleksić
Here's a subtle marble-like background for use on websites.
Source V. Hartikainen
A background pattern inspired by designs seen in 'Burghley. The Life of William Cecil', William Charlton, 1857.
Source Firkin
A seamless green background texture. The image is distributed under a Creative Commons License (like all of the images here).
Source V. Hartikainen
The image depicts a seamless pattern of Japanese Edo pattern called "kikkou-matsu" or "亀甲松" meaning " tortoiseshell-pinetree".The real pinetree is like this: https://jp.pinterest.com/pin/500744052301065077/
Source Yamachem
A playful triangle pattern with different shades of gray.
Source Dimitrie Hoekstra
Not the Rebel alliance, but a dark textured pattern.
Source Hendrik Lammers
Snap! It’s a pattern, and it’s not grayscale! Of course you can always change the color in Photoshop.
Source Atle Mo
Love me some light mesh on a Monday. Sharp.
Source Wilmotte Bastien
ZeroCC tileable stone texture, edited from pixabay. CC0
Source Sojan Janso
From a drawing in 'Handbook of the excursions proposed to be made by the Lincoln Diocesan Architectural Society, on the 27th and 28th of May, 1857', Edward Trollope, 1857.
Source Firkin
You know you can’t get enough of these linen-fabric-y patterns.
Source James Basoo
Zero CC tileable dry grass texture, photographed and made by me. CC0
Source Sojan Janso
The tile this is formed from can be retrieved in Inkscape by selecting the rectangle and using shift+alt+i
Source Firkin
This reminds me of Game Cube. A nice light 3D cube pattern.
Source Sander Ottens
Number 2 in a series of 5 beautiful patterns. Can be found in colors on the submitter’s website.
Source Janos Koos
A re-make of the Gradient Squares pattern.
Source Dimitar Karaytchev
Here's a brown background pattern with subtle stripes. I hope you'll like the color. If not, feel free to change it using an image editor, if you know how of course. Personally, I'm using GIMP to create these backgrounds.
Source V. Hartikainen
Prismatic Isometric Cube Extra Pattern No Background
Source GDJ