From a drawing in 'La Principauté de Liège et les Pays-Bas au XVIe siècle', Société des Bibliophiles Liégeois ,1887.
Source Firkin
Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
Formed by distorting the inside front cover of 'Diversæ insectarum volatilium : icones ad vivum accuratissmè depictæ per celeberrimum pictorem', Jacob Hoefnagel, 1630.
Source Firkin
Prismatic Abstract Geometric Background 2 No Black
Source GDJ
ZeroCC tileable wood boards texture, photographed and made by me. CC0
Source Sojan Janso
The tile this fill pattern is based on can be had by using shift+alt+i on the rectangle.
Source Firkin
A seamless background drawn in Paint.net and vectorised with Vector Magic. The starting point was a photograph of drinking straws from Pixabay.
Source Firkin
The rectangular tile this is based on can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin
To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
A free seamless background with pink spots.
Source V. Hartikainen
Submitted as a black pattern, I made it light and a few steps more subtle.
Source Andy
From a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Sharp pixel pattern looking like some sort of fabric.
Source Dmitry
A repeating gloomy background image. This one consists of a pattern of black chains layered on top of a dark textured background.
Source V. Hartikainen
This one is super crisp at 2X. Lined paper with some dust and scratches.
Source HQvectors
Zero CC tileable moss or lichen covered stone texture, edited from pixabay. CC0
Source Sojan Janso