Super simple but very nice indeed. Gray with vertical stripes.
Source Merrin Macleod
From a drawing in 'Art Embroidery', M.S. Lockwood and E. Glaister, 1878.
Source Firkin
The classic notebook paper with horizontal stripes.
Source Are Sundnes
Prismatic Abstract Background Design No Black
Source GDJ
This is a seamless pattern of a woody texture.The original image is here:https://pixabay.com/ja/users/ClassicallyPrinted-1302233/
Source Yamachem
A smooth mid-tone gray, or low contrast if you will, linen pattern.
Source Jordan Pittman
Used correctly, this could be nice. Used in a bad way, all hell will break loose.
Source Atle Mo
Greyscale version of a pattern that came out of playing with the 'light rays' plug-in for Paint.net
Source Firkin
Prismatic Geometric Tessellation Pattern 2 No Background
Source GDJ
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
A rusty grunge background for websites. Feel free to use it in your site's theme.
Source V. Hartikainen
To get the tile this is based on, select the rectangle in Inkscape and use shift-alt-i.
Source Firkin
Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
Alternative colour scheme. Not a pattern for fabrics, but one produced from a jpg of a stack of fabric items that was posted on Pixabay. The tile that this is based on can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
The name alone is awesome, but so is this sweet dark pattern.
Source Federica Pelzel
A tile-able background for websites with paper-like texture and a grid pattern layered on top of it.
Source V. Hartikainen
Classic vertical lines, in all its subtlety.
Source Cody L
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
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
A hint of orange color, and some crossed and embossed lines.
Source Adam Anlauf
Hexagonal dark 3D pattern. What more can you ask for?
Source Norbert Levajsics
Smooth Polaroid pattern with a light blue tint.
Source Daniel Beaton