Here's a repeatable texture that resembles a light green concrete wall or something similar.
Source V. Hartikainen
Remixed from a drawing in 'Chambéry à la fin du XIVe siècle', Timoleon Chapperon, 1863.
Source Firkin
A free seamless background image with a texture of dark red "canvas". It should look very nice on web sites.
Source V. Hartikainen
A light gray fabric pattern with faded vertical stripes.
Source V. Hartikainen
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
Drawn in Paint.net using the kaleidoscope plug-in and vectorised.
Source Firkin
Turn your site into a dragon with this great scale pattern.
Source Alex Parker
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
CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
Three shades of gray makes this pattern look like a small carbon fiber surface. Great readability even for small fonts.
Source Atle Mo
A light gray wall or floor (you decide) of concrete.
Source Atle Mo
Derived from an image that was uploaded to Pixabay by darkmoon1968
Source Firkin