I know there is one here already, but this is sexy!
Source Gjermund Gustavsen
This is the remix of an OCAL clipart called "Rain on Window" uploaded by "pagarmidna".Thanks.This is a seamless pattern of raindrops.
Source Yamachem
These dots are already worn for you, so you don’t have to.
Source Matt McDaniel
From a drawing in 'Art Embroidery', M.S. Lockwood and E. Glaister, 1878.
Source Firkin
A seamless pattern based on a rectangular tile that can be retrieved in Inkscape by selecting the rectangle and using shift-alt-i.
Source Firkin
A seamless texture of a rough concrete surface.
Source V. Hartikainen
Zero CC tileable hard cover cells, skin like, book texture. 4K, Scanned and made by me CC0
Source Sojan Janso
A seamless pattern the unit cell for which can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Remixed from a drawing in 'Maidenhood; or, the Verge of the Stream', Laura Jewry, 1876.
Source Firkin
A free seamless background image with abstract texture of green "curtain".
Source V. Hartikainen
This was formed by distorting an image of a background on Pixabay.
Source Firkin
Textured Red Brown Plastic, Free Background Pattern. Although there's already enough plastic in our lives, let's bring it to the web too.)
Source V. Hartikainen
And some more testing, this time with Seamless Studio. It’s Robots FFS!
Source Seamless Studio
To get the tile this is based on select the rectangle in Inkscape and use shift+alt+i
Source Firkin
A seamless background pattern of dark brown wood planks.
Source V. Hartikainen
Bigger is better, right? So here you have some large carbon fiber.
Source Factorio.us Collective
To get the tile this is based on select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Remixed from a raster on Pixabay that was uploaded by ArtsyBee.
Source Firkin
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
Alternative colour scheme for the original floral pattern.
Source Firkin