The starting point for this was a texture drawn with the 'Radial Colors' plug-in in Paint.net.
Source Firkin
Design drawn in Paint.net, vectorised using Vector Magic and finished in Inkscape.
Source Firkin
Prismatic Abstract Background Design
Source GDJ
CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
A version without colours blended together to give a different look.
Source Firkin
Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
Almost like little fish shells, or dragon skin.
Source Graphiste
White fabric looking texture with some nice random wave features.
Source Hendrik Lammers
Thin lines, noise and texture creates this crisp dark denim pattern.
Source Marco Slooten
From a drawing in 'At home', J. Sowerby, J. Crane and T. Frederick, 1881.
Source Firkin
ZeroCC tileable moss texture, photographed and made by me. CC0
Source Sojan Janso
Background pattern originally a PNG drawn in Paint.net
Source Firkin
Honestly, who does not like a little pipe and mustache?
Source Luca Errico
A light gray background pattern with seamless fabric-like texture and almost unnoticeable stripes.
Source V. Hartikainen
Number five from the same submitter, makes my job easy.
Source Dima Shiper
To get the tile this is based on select the rectangle in Inkscape and use shift-alt-i
Source Firkin
Luxury pattern, looking like it came right out of Paris.
Source Daniel Beaton
Prismatic Geometric Pattern Background 2 No Black
Source GDJ
A seamless pattern with wide vertical stripes colored in pale yellow.
Source V. Hartikainen
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
Sharp pixel pattern looking like some sort of fabric.
Source Dmitry
Remixed from an image on Pixabay uploaded by Prawny
Source Firkin
A seamless background pattern of dark brown wood planks.
Source V. Hartikainen
CC0 and seamless wellington boot pattern.
Source SliverKnight