Because I love dark patterns, here is Brushed Alum in a dark coating.
Source Tim Ward
A seamless chequerboard pattern formed from a tile that can be had in Inkscape by selecting the rectangle and using shift+alt+i. Alternative colour scheme.
Source Firkin
After 1 comes 2, same but different. You get the idea.
Source Hendrik Lammers
Medium gray fabric pattern with 45-degree lines going across.
Source Atle Mo
The classic notebook paper with horizontal stripes.
Source Are Sundnes
Prismatic Geometric Pattern Background No Black
Source GDJ
A pattern formed from a squared tile. The tile can be accessed in Inkscape by selecting the rectangle and using shift-alt-i.
Source Firkin
A blue background wallpaper for websites. It has a seamless texture with vertical stripes. It looks quite nice not only when using as a tiled background on websites, but also on computer desktops.
Source V. Hartikainen
A car pattern?! Can it be subtle? I say yes!
Source Radosław Rzepecki
A seamless pattern the unit cell for which can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin
Prismatic Curved Diamond Pattern 2 No Background
Source GDJ
From a drawing in 'Sun Pictures of the Norfolk Broads', Ernest Suffling, 1892.
Source Firkin
The square tile this is based on can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin
Looks like a technical drawing board: small squares forming a nice grid.
Source We Are Pixel8
Formed by heavily distorting part of a an image of a fish uploaded to Pixabay by GLady
Source Firkin
More carbon fiber for your collections. This time in white or semi-dark gray.
Source Badhon Ebrahim
Real snow that tiles, not easy. This is not perfect, but an attempt.
Source Atle Mo
If you want png files of this u can download them here : viscious-speed.deviantart.com/gallery/27635117
Source Viscious-Speed
There are quite a few grid patterns, but this one is a super tiny grid with some dust for good measure.
Source Dominik Kiss
I guess this one is inspired by an office. A dark office.
Source Andrés Rigo.
Crossing lines with a subtle emboss effect on a dark background.
Source Stefan Aleksić