The classic 45-degree diagonal line pattern, done right.
Source Jorick van Hees
Non-seamless pattern drawn in Paint.net and vectorised in Vector Magic.
Source Firkin
Inspired by a drawing in 'Poems', James Smith, 1881.
Source Firkin
Prismatic Triangular Background Design Mark II 5
Source GDJ
From a drawing in 'Sun Pictures of the Norfolk Broads', Ernest Suffling, 1892.
Source Firkin
Prismatic Abstract Background Design
Source GDJ
A seamless pattern recreated from an image on Pixabay. It is reminiscent of parquet flooring and is formed from a square tile, which can be recovered in Inkscape by selecting the ungrouped rectangle and using shift-alt-I together.
Source Firkin
A light gray wall or floor (you decide) of concrete.
Source Atle Mo
Lovely pattern with splattered vintage speckles.
Source David Pomfret
A seamless pattern formed from miutopia's cakes on a tablecloth.
Source Firkin
Just like your old suit, all striped and smooth.
Source Alex Berkowitz
A pattern derived from repeating unit cells each derived from part of a fractal rendering in paint.net.
Source Firkin
Nice and simple crossed lines in dark gray tones.
Source Stefan Aleksić
Derived from a PNG that was uploaded to Pixabay by nutkitten
Source Firkin
Psychedelic Geometric Background No Black
Source GDJ
Might not be super subtle, but quite original in its form.
Source Alex Smith
If you’re sick of the fancy 3D, grunge and noisy patterns, take a look at this flat 2D brick wall.
Source Listvetra
From a drawing in 'In an Enchanted Island', William Mallock, 1892.
Source Firkin
Super simple but very nice indeed. Gray with vertical stripes.
Source Merrin Macleod
Inspired by a 1930s wallpaper pattern I saw on TV.
Source Firkin
Here I have tried to create something that would look like maple wood. Not sure how well it's turned out, but at least it looks like wood.
Source V. Hartikainen
CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
From a drawing in 'Maidenhood; or, the Verge of the Stream', Laura Jewry, 1876.
Source Firkin