Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
From a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
There are many carbon patterns, but this one is tiny.
Source Designova
A beautiful dark padded pattern, like an old classic sofa.
Source Chris Baldie
Gold Triangular Seamless Pattern No Background
Source GDJ
Seamless pattern formed from a tile that can be had 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
Sort of like the Photoshop transparent background, but better!
Source Alex Parker
Same as gray sand but lighter. A sandy pattern with small light dots, and some angled strokes.
Source Atle Mo
This is indeed a bit strange, but here’s to the crazy ones!
Source Christopher Buecheler
This light blue background pattern is quite pleasing to the eye, it consists of a tiny rough grid pattern, which is seamless by design. That's it, if you like the color, you can use this seamless pattern in a web design without making any further modifications to it.
Source V. Hartikainen
As simple and subtle as it gets. But sometimes that’s just what you want.
Source Designova
Remixed from a drawing in 'Line and form', Walter Crane, 1914.
Source Firkin
Prismatic Hypnotic Pattern No Background
Source GDJ
Dark squares with some virus-looking dots in the grid.
Source Hugo Loning
A beautiful dark padded pattern, like an old classic sofa.
Source Chris Baldie
You know you love wood patterns, so here’s one more.
Source Richard Tabor
Some rectangles, a bit of dust and grunge, plus a hint of concrete.
Source Atle Mo
A seamless web background with texture of aged grid paper.
Source V. Hartikainen