Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
If you want png files of this u can download them here : viscious-speed.deviantart.com/gallery/27635117
Source Viscious-Speed
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 Geometric Pattern Background 2 No Black
Source GDJ
From a drawing in 'Hundert Jahre in Wort und Bild', S. Stefan, 1899.
Source Firkin
I know there is one here already, but this is sexy!
Source Gjermund Gustavsen
I have no idea how to describe this one, but it’s light and delicate.
Source JBasoo
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
From a drawing in 'Artists and Arabs', Henry Blackburn, 1868
Source Firkin
The image depicts a seamless pattern of a snow crystal.I referred to a book called ”sekka-zusetsu” or "雪華図説" which means an illustrated explanation about snow crystals.This book was published in 1832 (天保3年) or Edo period.For more about "雪華図説",see here:dl.ndl.go.jp/info:ndljp/pid/2536975
Source Yamachem
A seamless pattern from a tile made from a jpg on Pixabay. To get the tile select the rectangle in Inkscape and use shift-alt-i.
Source Firkin
Fake or not, it’s quite luxurious.
Source Factorio.us Collective
Some rectangles, a bit of dust and grunge, plus a hint of concrete.
Source Atle Mo
Old China with a modern twist, take two.
Source Adam Charlts
Colour version of the original pattern.
Source Firkin
From a drawing in 'Cowdray: the history of a great English House', Julia Roundell, 1884.
Source Firkin
Remixed from a drawing in 'The Canadian horticulturist', 1892
Source Firkin
Remixed from a drawing in 'Paul's Sister', Frances Peard, 1889.
Source Firkin
A seamless pattern formed from a square tile. The tile can be retrieved by selecting the rectangle in Inkscape and using shift-alt-i.
Source Firkin
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin