The green fibers pattern will work very well in grayscale as well.
Source Matteo Di Capua
To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
First pattern tailor-made for Retina, with many more to come. All the old ones are upscaled, in case you want to re-download.
Source Atle Mo
Remixed from a drawing in 'Maidenhood; or, the Verge of the Stream', Laura Jewry, 1876.
Source Firkin
Thin lines, noise and texture creates this crisp dark denim pattern.
Source Marco Slooten
From a drawing in 'At home', J. Sowerby, J. Crane and T. Frederick, 1881.
Source Firkin
Remixed from a drawing in 'Line and form', Walter Crane, 1914.
Source Firkin
A seamless pale yellow paper background with a pattern of animal tracks.
Source V. Hartikainen
Drawn in Paint.net using the kaleidoscope plug-in and vectorised.
Source Firkin
Seamless Olive Green Web Background Image
Source V. Hartikainen
An abstract texture of water. It's not perfect, but will do. You may download if you like it.
Source V. Hartikainen
A seamless dark leather-like background texture with diagonal lines that look like stitches.
Source V. Hartikainen
Seamless , tileable CC-0 texture. Created by my own, feel free to use wherever you want!
Source Linolafett
Prismatic Geometric Tessellation Pattern 4 No Background
Source GDJ
From a drawing in 'Jardyne's Wife', Charles Wills, 1891.
Source Firkin
A background pattern with blue on white vertical stripes.
Source V. Hartikainen
Here's a new paper-like background for free use on personal and commercial projects (this applies to all background patterns here).
Source V. Hartikainen
Colour version that is close to the original drawing uploaded to Pixabay by pencilparker.
Source Firkin
A new one called white wall, not by me this time.
Source Yuji Honzawa
No idea what Nistri means, but it’s a crisp little pattern nonetheless.
Source Markus Reiter
A seamless pattern formed from a sports car on clker.com. To get the tile, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Prismatic 3D Isometric Tessellation Pattern 6
Source GDJ
The tile this is formed from can be retrieved in Inkscape by selecting the rectangle and using shift+alt+i
Source Firkin
Here's a camo print with more tan and less green, such as might be used in a desert scenario. This is tileable, so it can be used as a wallpaper or background.
Source Eady