Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
Derived from an image that was uploaded to Pixabay by Kaz
Source Firkin
Remixed from a drawing in 'Line and form', Walter Crane, 1914.
Source Firkin
This ladies and gentlemen, is texturetastic! Love it.
Source Adam Pickering
White fabric looking texture with some nice random wave features.
Source Hendrik Lammers
A version without colours blended together to give a different look.
Source Firkin
Super subtle indeed, a medium gray pattern with tiny dots in a grid.
Source Designova
It’s like Shine Dotted’s sister, only rotated 45 degrees.
Source mediumidee
Sweet and subtle white plaster with hints of noise and grunge.
Source Phil Maurer
A seamless pattern from a tile drawn in Paint.net and vectorised in Vector Magic
Source Firkin
A seamless pattern from a tile drawn in Paint.net and vectorised in Vector Magic
Source Firkin
The unit cell for this seamless pattern can be had in Inkscape by selecting the rectangle and using shift+alt+i.
Source Firkin
Remixed from a PNG that was uploaded to Pixabay by gingertea
Source Firkin
As far as fabric patterns goes, this is quite crisp.
Source Heliodor Jalba
Prismatic Groovy Concentric Background 4
Source GDJ
A seamless pattern formed from a square tile. The tile can be retrieved by selecting the rectangle in Inkscape and using shift-alt-I. Version with black background.
Source Firkin
CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
White circles connecting on a light gray background.
Source Mark Collins
Tile available in Inkscape using shift-alt-i on the selected rectangle
Source Firkin
Actually remixed from a pattern on Pixabay. But then noticed a very similar one on Openclipart.org uploaded by btj51q2.
Source Firkin