Dark, lines, noise, tactile. You get the drift.
Source Anatoli Nicolae
Horizontal and vertical lines on a light gray background.
Source Adam Anlauf
Prismatic Rounded Squares Grid 4 No Background
Source GDJ
White handmade paper pattern with small bumps.
Source Marquis
Might not be super subtle, but quite original in its form.
Source Alex Smith
From a drawing in 'Line and form', Walter Crane, 1914.
Source Firkin
This seamless pattern consists of a blue grid on a yellow background.
Source V. Hartikainen
From a drawing in 'Royal Ramsgate', James Simson, 1897.
Source Firkin
CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
Like the name suggests, this background image consists of a pattern of dark bricks. It may be an option for you, if you are looking for something that looks like a brick wall for use as a background on web pages. It's not a masterpiece, but looks pretty nice when is tiled.
Source V. Hartikainen
Seamless , tileable CC-0 texture. Created by my own, feel free to use wherever you want!
Source Linolafett
Gold Triangular Seamless Pattern No Background
Source GDJ
Sharp diamond pattern. A small 24x18px tile.
Source Tom Neal
Sharp pixel pattern looking like some sort of fabric.
Source Dmitry
Imagine you zoomed in 1000X on some fabric. But then it turned out to be a skeleton!
Source Angelica
Prismatic Geometric Pattern Background 2 No Black
Source GDJ
An abstract pale yellow paper-like background with stains colored in yellow and green.
Source V. Hartikainen
Sweet and subtle white plaster with hints of noise and grunge.
Source Phil Maurer
Green Background Pattern
Source V. Hartikainen
Seamless pattern formed from a tile that can be had by selecting the rectangle in Inkscape and using shift-alt-i.
Source Firkin
Remixed from a drawing in 'An Index to Deering's Nottinghamia Vetus et Nova', Rupert Chicken, 1899. The unit tile can be had by selecting the rectangle in Inkscape and using shift-alt-i
Source Firkin
More leather, and this time it’s bigger! You know, in case you need that.
Source Elemis
From a drawing in 'In an Enchanted Island', William Mallock, 1892.
Source Firkin