A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
Subtle scratches on a light gray background.
Source Andrey Ovcharov
Remixed from a drawing in 'Chambéry à la fin du XIVe siècle', Timoleon Chapperon, 1863.
Source Firkin
The unit cell for this seamless pattern can be had in Inkscape by selecting the rectangle and using shift+alt+i.
Source Firkin
This one is amazing, truly original. Go use it!
Source Viahorizon
A seamless pattern formed from a square tile. The tile can be retrieved by selecting the rectangle in Inkscape and using shift-alt-I.
Source Firkin
After 1 comes 2, same but different. You get the idea.
Source Hendrik Lammers
Paper model of a tetrahedron. Modelo de papel de um tetraedro.
Source laobc
Inspired by a pattern found in 'A General History of Hampshire, or the County of Southampton, including the Isle of Wight', Bernard Woodwood, 1861
Source Firkin
This background texture resembles stone. It may be used as a background on web pages or on some of their html elements (header, borders, menu bar, etc.). Just modify it for your needs.
Source V. Hartikainen
Based on an image that was uploaded to Pixabay by devanath
Source Firkin
Pixel by pixel, sharp and clean. Very light pattern with clear lines.
Source M.Ashok
A seamless texture of a rough concrete surface.
Source V. Hartikainen
Made by distorting a simple pattern using the 'sin waves' plugin for Paint.net and vectorising in Vector Magic
Source Firkin
The green fibers pattern will work very well in grayscale as well.
Source Matteo Di Capua
Here's a subtle marble-like background for use on websites.
Source V. Hartikainen
This one could be the shirt of a golf player. Angled lines in different thicknesses.
Source Olivier Pineda
Sharp pixel pattern, just like the good old days.
Source Paridhi