Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
From a drawing in 'Heroes of North African Discovery', Nancy Meugens, 1894.
Source Firkin
Colorful Floral Background 3 No Black
Source GDJ
This one is rather fun and playful. The 2X could be used at 1X too!
Source Welsley
A good starting point for a cardboard pattern. This would work well in a variety of colors.
Source Atle Mo
A light gray fabric pattern with faded vertical stripes.
Source V. Hartikainen
First pattern tailor-made for Retina, with many more to come. All the old ones are upscaled, in case you want to re-download.
Source Atle Mo
Produced using the clouds, flames and glass blocks plug-ins in Paint.net and the resulting .PNG vectorised with Vector Magic.
Source Firkin
Prismatic Polka Dots 3 No Background
Source GDJ
To get the tile this is based on select the rectangle in Inkscape and use shift-alt-i
Source Firkin
Remixed from a drawing in 'Line and form', Walter Crane, 1914.
Source Firkin
High detail stone wall with minor cracks and specks.
Source Projecteightyfive
You can never get enough of these tiny pixel patterns with sharp lines.
Source Designova
Seamless , tileable CC-0 texture. Created by my own, feel free to use wherever you want!
Source Linolafett
Beautiful dark noise pattern with some dust and grunge.
Source Vincent Klaiber
From a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
A free seamless background image with abstract texture of green "curtain".
Source V. Hartikainen
You don’t see many mid-tone patterns here, but this one is nice.
Source Joel Klein
Alternative colour scheme for the original floral pattern.
Source Firkin
Used a cherry by doctormo to make this seamless pattern
Source Firkin