Here's a subtle marble-like background for use on websites.
Source V. Hartikainen
More in the paper realm, this time with fibers.
Source Jorge Fuentes
Seamless , tileable CC-0 texture. Created by my own, feel free to use wherever you want!
Source Linolafett
A seamless pattern with a unit cell drawn as a bitmap in Paint.net and vectorized in Vector Magic.
Source Firkin
Abstract Stars Geometric Pattern Prismatic No Background
Source GDJ
Nicely crafted paper pattern, although a bit on the large side (500x593px).
Source Blaq Annabiosis
Some rectangles, a bit of dust and grunge, plus a hint of concrete.
Source Atle Mo
He influenced us all. “Don’t be sad because it’s over. Smile because it happened.”
Source Atle Mo
This was formed by distorting an image of a background on Pixabay.
Source Firkin
From a drawing in 'Artists and Arabs', Henry Blackburn, 1868
Source Firkin
Adapted from an image that was uploaded to Pixabay by Anerma.
Source Firkin
Colorful Floral Background 3 No Black
Source GDJ
Recreated from a pattern found in 'Az Osztrák-Magyar Monarchia irásban és képben', 1882. To get the tile this is based on select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Sometimes you just need the simplest thing.
Source Fabricio
From a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
A seamless pale yellow paper background with a pattern of animal tracks.
Source V. Hartikainen
One more from Badhon, sharp horizontal lines making an embossed paper feeling.
Source Badhon Ebrahim
One more sharp little tile for you. Subtle circles this time.
Source Blunia
A nice and simple gray stucco material. Great on its own, or as a base for a new pattern.
Source Bartosz Kaszubowski
Colored maple leaves scattered on a surface. This is tileable, so it can be used as a background or wallpaper.
Source Eady
An abstract pale yellow paper-like background with stains colored in yellow and green.
Source V. Hartikainen
A free seamless background image with abstract texture of green "curtain".
Source V. Hartikainen
An emulated “transparent” background pattern, like that of all kinds of computer graphics software.
Source AdamStanislav
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin