Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
Looks like an old wall. I guess that’s it then?
Source Viahorizon
Small gradient crosses inside 45-degree boxes, or bigger crosses if you will.
Source Wassim
The rectangular tile this is based on can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin
A grid of squares with green colours. Since the colours are randomly distributed it is automatically seamless.
Source Firkin
A heavy dark gray base, some subtle noise and a 45-degree grid makes this look like a pattern with a tactile feel to it.
Source Atle Mo
Dead simple but beautiful horizontal line pattern.
Source Fabian Schultz
Zero CC tillable hard cover red book with X shape marks. Scanned and made by me.
Source Sojan Janso
Prismatic Hexagonalism Pattern No Background
Source GDJ
This yellow background consists of a pattern of glossy gold buttons arranged in polka dot style on a seamless texture. Here's a pale yellow background pattern. Feel free to use it for your needs!
Source V. Hartikainen
Remixed from a drawing in 'Paul's Sister', Frances Peard, 1889.
Source Firkin
Sharp but soft triangles in light shades of gray.
Source Pixeden
A monochrome pattern from a tile that can be had by selecting the rectangle in Inkscaope and using shift+alt+i
Source Firkin
Prismatic Geometric Pattern Background 2 No Black
Source GDJ
Remixed from a drawing in 'An Index to Deering's Nottinghamia Vetus et Nova', Rupert Chicken, 1899. The unit tile can be had by selecting the rectangle in Inkscape and using shift-alt-i
Source Firkin
I took the liberty of using Dmitry’s pattern and made a version without perforation.
Source Atle Mo
A seamless background texture of old cardboard.
Source V. Hartikainen
A seamless pattern the starting point for which was a 'light rays' rendering in Paint.net.
Source Firkin
I love these crisp, tiny, super subtle patterns.
Source Badhon Ebrahim