Might not be super subtle, but quite original in its form.
Source Alex Smith
A pattern derived from repeating unit cells each derived from part of a fractal rendering in paint.net.
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
Prismatic Polka Dots Mark II No Background
Source GDJ
Seamless pattern the tile for which can be had by using shift-alt-I on the selected rectangle in Inkscape.
Source Firkin
Prismatic Groovy Concentric Background 4 No Black
Source GDJ
CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
Vector version of a png that was uploaded to Pixabay by pencilparker
Source Firkin
Prismatic Geometric Tessellation Pattern 3 No Background
Source GDJ
Prismatic Rounded Squares Grid 3 No Background
Source GDJ
Remixed from a drawing in 'Works. Popular edition', John Ruskin, 1886.
Source Firkin
From a drawing in 'Friend or Fortune? The story of a strange year', Robert Overton, 1897.
Source Firkin
A light gray fabric pattern with faded vertical stripes.
Source V. Hartikainen