Looks like a technical drawing board: small squares forming a nice grid.
Source We Are Pixel8
A re-make of the Gradient Squares pattern.
Source Dimitar Karaytchev
Made by distorting a simple pattern using the 'sin waves' plugin for Paint.net and vectorising in Vector Magic
Source Firkin
Dark Tile-able Grunge Texture. I think this texture can be classified as grunge. It's free and seamless, as always.
Source V. Hartikainen
More in the paper realm, this time with fibers.
Source Jorge Fuentes
An alternative colour scheme for the original background.
Source Firkin
I have no idea how to describe this one, but it’s light and delicate.
Source JBasoo
Prismatic Rounded Squares Grid 3 No Background
Source GDJ
A background pattern inspired by designs seen in 'Burghley. The Life of William Cecil', William Charlton, 1857.
Source Firkin
A huge one at 800x600px. Made from a photo I took going home after work.
Source Atle Mo
Simple wide squares with a small indent. Fits all.
Source Petr Šulc.
An alternative colour scheme to the original seamless pattern.
Source Firkin
A light gray background pattern with seamless fabric-like texture and almost unnoticeable stripes.
Source V. Hartikainen
Tiny little fibers making a soft and sweet look.
Source Badhon Ebrahim
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
Prismatic Geometric Pattern Background 2
Source GDJ
One of the few full-color patterns here, but this one was just too good to pass up.
Source Alexey Usoltsev
Abstract Ellipses Background Grayscale
Source GDJ
Remixed from a PNG that was uploaded to Pixabay by k_jprather
Source Firkin
Vector version of a png that was uploaded to Pixabay by pencilparker
Source Firkin
Dark wooden pattern, given the subtle treatment. based on texture from Cloaks. https://cloaks.deviantart.com
Source Atle Mo
Prismatic Groovy Concentric Background 2 No Black
Source GDJ
On a large canvas you can see it tiling, but used on smaller areas, it’s beautiful.
Source Paul Phönixweiß