A grid of squares with green colours. Since the colours are randomly distributed it is automatically seamless.
Source Firkin
Prismatic Hypnotic Pattern 2 No Background
Source GDJ
Sharp diamond pattern. A small 24x18px tile.
Source Tom Neal
If you want png files of this u can download them here : viscious-speed.deviantart.com/gallery/27635117
Source Viscious-Speed
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
More in the paper realm, this time with fibers.
Source Jorge Fuentes
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
Not even 1kb, but very stylish. Gray thin lines.
Source Struck Axiom
The image depicts a seamless pattern made using a bird's face.
Source Yamachem
Because I love dark patterns, here is Brushed Alum in a dark coating.
Source Tim Ward
Tiny, tiny 3D cubes. Reminds me of the good old pattern from k10k.
Source Etienne Rallion
ZeroCC tileable stone texture, edited from pixabay. CC0
Source Sojan Janso
You could get a bit dizzy from this one, but it might come in handy.
Source Dertig Media
This background pattern contains a seamless texture of bark. It's not very realistic, but I think it looks quite nice.
Source V. Hartikainen
I took the liberty of using Dmitry’s pattern and made a version without perforation.
Source Atle Mo
To get the tile this is based on, select the rectangle in Inkscape and use shift-alt-i.
Source Firkin
A grayscale fabric pattern with vertical lines of stitch holes.
Source V. Hartikainen
Sharp but soft triangles in light shades of gray.
Source Pixeden
A new one called white wall, not by me this time.
Source Yuji Honzawa
Prismatic Polka Dots Mark II 2 No Background
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.
Source Firkin
Here's a bluish gray striped background pattern for use on web sites.
Source V. Hartikainen
A comeback for you: the popular Escheresque, now in black.
Source Patten
Remixed from a JPG that was uploaded to Pixabay by theasad121
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