A seamless chequerboard pattern formed from a tile that can be had in Inkscape by selecting the rectangle and using shift+alt+i. Alternative colour scheme.
Source Firkin
A seamless dark leather-like background texture with diagonal lines that look like stitches.
Source V. Hartikainen
Just to prove my point, here is a slightly modified dark version.
Source Atle Mo
A free seamless background image with a texture of dark red "canvas". It should look very nice on web sites.
Source V. Hartikainen
Prismatic Groovy Concentric Background 5
Source GDJ
Derived from a drawing in 'Historiske Afhandlinger', Adolf Jorgensen, 1898.
Source Firkin
Redrawn based on a drawing in 'По Сѣверо-Западу Россіи' Konstantin Sluchevsky, 1897.
Source Firkin
I took the liberty of using Dmitry’s pattern and made a version without perforation.
Source Atle Mo
A very dark asfalt pattern based off of a photo taken with my iPhone.
Source Atle Mo
It’s like Shine Dotted’s sister, only rotated 45 degrees.
Source mediumidee
Remixed from a drawing in 'A Child of the Age', Francis Adams, 1894.
Source Firkin
A seamless tessellation pattern. To get the tile this is formed from, select the pattern in Inkscape and use shift+alt+i.
Source Firkin
Prismatic Polka Dots Mark II 2 No Background
Source GDJ
This was formed by distorting an image of a background on Pixabay.
Source Firkin
I know there is one here already, but this is sexy!
Source Gjermund Gustavsen
Used in small doses, this could be a nice subtle pattern. Used on a large surface, it’s dirty!
Source Paul Reulat
Prismatic Abstract Line Art Pattern Background 2
Source GDJ
From a drawing in 'At home', J. Sowerby, J. Crane and T. Frederick, 1881.
Source Firkin