A seamless pattern with a unit cell drawn as a bitmap in Paint.net and vectorized in Vector Magic.
Source Firkin
This one is so simple, yet so good. And you know it. Has to be in the collection.
Source Gluszczenko
From a drawing in 'Cassell's Library of English Literature', Henry Morley, 1883.
Source Firkin
A dark background pattern/texture of a dimpled metal plate.
Source V. Hartikainen
A re-make of the Gradient Squares pattern.
Source Dimitar Karaytchev
Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
Derived from elements found in a floral ornament drawing on Pixabay.
Source Firkin
There are quite a few grid patterns, but this one is a super tiny grid with some dust for good measure.
Source Dominik Kiss
Prismatic Abstract Background Design
Source GDJ
To get the tile this is based on, select the rectangle in Inkscape and use shift-alt-i.
Source Firkin
From a drawing in 'Art Embroidery', M.S. Lockwood and E. Glaister, 1878.
Source Firkin
Prismatic Snowflakes Pattern 2 No Background
Source GDJ
The name tells you it has curves. Oh yes, it does!
Source Peter Chon
Abstract Arbitrary Geometric Background derived from an image on Pixabay.
Source GDJ
From a drawing in 'Sun Pictures of the Norfolk Broads', Ernest Suffling, 1892.
Source Firkin
Remixed from a drawing in 'Hungary. A guide book. By several authors', 1890.
Source Firkin
Zero CC tillable hard cover red book with X shape marks. Scanned and made by me.
Source Sojan Janso
CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight