The green fibers pattern will work very well in grayscale as well.
Source Matteo Di Capua
Semi-light fabric pattern made out of random pixels in shades of gray.
Source Atle Mo
Seamless pattern formed from a tile that can be extracted by selecting the rectangle in Inkscape and using shift-alt-i.
Source Firkin
From a drawing in 'Hubert Montreuil, or the Huguenot and the Dragoon', Francisca Ouvry, 1873.
Source Firkin
This is a remix of "flower seamless pattern".I rotated the original image by 90 degrees.This is a seamless pattern of flowers.These horizontal wavy lines are one of Edo patterns which is called "tatewaku or tachiwaku or 立湧" that represents uprising steam or vapor.
Source Yamachem
Might not be super subtle, but quite original in its form.
Source Alex Smith
From a drawing in 'Artists and Arabs', Henry Blackburn, 1868.
Source Firkin
A seamless marble-like texture colored in light blue.
Source V. Hartikainen
This is so subtle I hope you can see it! Tweak at will.
Source Alexandre Naud
CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
Cubes as far as your eyes can see. You know, because they tile.
Source Jan Meeus
Super simple but very nice indeed. Gray with vertical stripes.
Source Merrin Macleod
A seamless background colored in pale orange. It has a paper like texture with diagonal grid pattern.
Source V. Hartikainen
The tile can be had by selecting the rectangle in Inkscape and using shift+alt+i. Remixed from a drawing in 'Flowers of Song', Frederick Weatherly, 1895.
Source Firkin
Embossed lines and squares with subtle highlights.
Source Alex Parker
A seamless texture of worn out "cardboard".
Source V. Hartikainen
A seamless pattern of "sewn stripes" colored in light gray.
Source V. Hartikainen
Here's a subtle marble-like background for use on websites.
Source V. Hartikainen
This ladies and gentlemen, is texturetastic! Love it.
Source Adam Pickering