From a drawing in 'The Quiver of Love', Walter Crane, 1876
Source Firkin
Seamless pattern the tile for which can be had by using shift-alt-I on the selected rectangle in Inkscape.
Source Firkin
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
Inspired by a pattern I saw in a 19th century book. This seamless pattern was created from a square tile. To get the tile, select the pattern in Inkscape and use shift+alt+i.
Source Firkin
And some more testing, this time with Seamless Studio. It’s Robots FFS!
Source Seamless Studio
Prismatic Geometric Pattern Background No Black
Source GDJ
Drawn in Paint.net and vectorised in Vector Magic
Source Firkin
Seamless Dark Grunge Texture. Here's a new grunge texture for use as a background.
Source V. Hartikainen
Light square grid pattern, great for a “DIY projects” sort of website, maybe?
Source Rafael Almeida
To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Light gray grunge wall with a nice texture overlay.
Source Adam Anlauf
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
From a drawing of the coat of arms of the Ottoman Empire on Wikimedia.
Source Firkin
A light gray fabric pattern with faded vertical stripes.
Source V. Hartikainen
The unit cell for this seamless pattern can be had in Inkscape by selecting the rectangle and using shift+alt+i.
Source Firkin
Remixed from a raster on Pixabay, that was uploaded by ArtsyBee.
Source Firkin
New paper pattern with a slightly organic feel to it, using some thin threads.
Source Atle Mo
Here's a subtle marble-like background for use on websites.
Source V. Hartikainen
If you’re sick of the fancy 3D, grunge and noisy patterns, take a look at this flat 2D brick wall.
Source Listvetra