This is indeed a bit strange, but here’s to the crazy ones!
Source Christopher Buecheler
Submitted as a black pattern, I made it light and a few steps more subtle.
Source Andy
To get the tile this is formed from select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Formed by distorting an image on Pixabay that was uploaded by gustavorezende. To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
A white version of the very popular linen pattern.
Source Ant Ekşiler
The square tile this is based on can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin
A seamless green background texture. The image is distributed under a Creative Commons License (like all of the images here).
Source V. Hartikainen
Beautiful dark noise pattern with some dust and grunge.
Source Vincent Klaiber
The image depicts a Japanese Edo pattern called "kanoko or 鹿の子" meaning "fawn" which has a fur with small white spots.
Source Yamachem
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
A seamless texture of an abstract wall colored in shades of light orange brown.
Source V. Hartikainen
This one is amazing, truly original. Go use it!
Source Viahorizon
Clean and crisp lines all over the place. Wrap it up with this one.
Source Dax Kieran
Uses spirals from Pixabay. To get the basic tile select the rectangle in Inkscape and use shift+alt+i
Source Firkin
From a drawing in 'Les Chroniqueurs de l'Histoire de France depuis les origines jusqu'au XVIe siècle', Henriette Witt, 1884.
Source Firkin
Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
Seamless SVG vector and JPG backgrounds with faded diagonal stripes. The colors are editable.
Source V. Hartikainen
Dead simple but beautiful horizontal line pattern.
Source Fabian Schultz
Alternative colour scheme. Not a pattern for fabrics, but one produced from a jpg of a stack of fabric items that was posted on Pixabay. The tile that this is based on can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Seamless pattern formed from a tile that can be extracted by selecting the rectangle in Inkscape and using shift-alt-i.
Source Firkin
Plywood Web Background background image for use in web design.
Source V. Hartikainen
Prismatic Basic Pattern 2 No Background
Source GDJ