From a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
As far as fabric patterns goes, this is quite crisp.
Source Heliodor Jalba
From a drawing in 'Gately's World's Progress', Charles Beale, 1886.
Source Firkin
This is indeed a bit strange, but here’s to the crazy ones!
Source Christopher Buecheler
Sounds French. Some 3D square diagonals, that’s all you need to know.
Source Graphiste
From a drawing in 'Sun Pictures of the Norfolk Broads', Ernest Suffling, 1892.
Source Firkin
Made by distorting a simple pattern using the 'sin waves' plugin for Paint.net and vectorising in Vector Magic
Source Firkin
Just to prove my point, here is a slightly modified dark version.
Source Atle Mo
Prismatic Abstract Line Art Pattern Background 2
Source GDJ
This one needs to be used in small areas; you can see it repeat.
Source Luca
A simple example on using clones. You can generate a nice base for a pattern fill quickly with it.
Source Lazur URH
You can never get enough of these tiny pixel patterns with sharp lines.
Source Designova
From a drawing in 'Les Chroniqueurs de l'Histoire de France depuis les origines jusqu'au XVIe siècle', Henriette Witt, 1884.
Source Firkin
The green fibers pattern will work very well in grayscale as well.
Source Matteo Di Capua
Used the 6th circle pattern designed by Viscious-Speed to create a print that can be used for card making or scrapbooking. Save as a PDF file for the best printing option.
Source Lovinglf
From a drawing in 'Hundert Jahre in Wort und Bild', S. Stefan, 1899.
Source Firkin
Colorful Floral Background No Black
Source GDJ
Vector version of a JPG that was uploaded to Pixabay by theasad121
Source Firkin
Prismatic Abstract Background Design No Black
Source GDJ
Dark, square, clean and tidy. What more can you ask for?
Source Jaromír Kavan
A seamless pattern from a tile drawn in Paint.net and vectorised in Vector Magic
Source Firkin
A bit of scratched up grayness. Always good.
Source Dmitry