There are quite a few grid patterns, but this one is a super tiny grid with some dust for good measure.
Source Dominik Kiss
After 1 comes 2, same but different. You get the idea.
Source Hendrik Lammers
There are many carbon patterns, but this one is tiny.
Source Designova
CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
Remixed from a drawing in 'Works. Popular edition', John Ruskin, 1886.
Source Firkin
Formed from a tile based on a drawing from 'Viaggi d'un artista nell'America Meridionale', Guido Boggiani, 1895.
Source Firkin
A light gray wall or floor (you decide) of concrete.
Source Atle Mo
A seamlessly tile-able grunge background image.
Source V. Hartikainen
A seamless background drawn in Paint.net and vectorised with Vector Magic. The starting point was a photograph of drinking straws from Pixabay.
Source Firkin
An interesting dark spotted pattern at an angle.
Source Hendrik Lammers
Free tiled background with colorful stripes and white splatter.
Source V. Hartikainen
A seamless pattern created from a square tile. To get the tile, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
ZeroCC tileable mossy (lichen) stone texture, edited from pixabay. CC0
Source Sojan Janso
This seamless pattern consists of a blue grid on a yellow background.
Source V. Hartikainen
This is a hot one. Small, sharp and unique.
Source GraphicsWall
Looks like a technical drawing board: small squares forming a nice grid.
Source We Are Pixel8
A simple example on using clones. You can generate a nice base for a pattern fill quickly with it.
Source Lazur URH
Horizontal and vertical lines on a light gray background.
Source Adam Anlauf
To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
A fun-looking elastoplast/band-aid pattern. A hint of orange tone in this one.
Source Josh Green