A seamless green background texture. The image is distributed under a Creative Commons License (like all of the images here).
Source V. Hartikainen
This one takes you back to math class. Classic mathematic board underlay.
Source Josh Green
A dark one with geometric shapes and dotted lines.
Source Mohawk Studios
The rectangular tile this is based on can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin
One can never have too few rice paper patterns, so here is one more.
Source Atle Mo
A seamless background pattern of dark brown wood planks.
Source V. Hartikainen
Like the name says, light and gray, with some small dots and circles.
Source Brenda Lay
To get the tile this is formed from select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Remixed from a drawing in 'Maidenhood; or, the Verge of the Stream', Laura Jewry, 1876.
Source Firkin
A background pattern with blue on white vertical stripes.
Source V. Hartikainen
This background image has seamless texture that resembles a surface of gray stone.
Source V. Hartikainen
More leather, and this time it’s bigger! You know, in case you need that.
Source Elemis
Prismatic Geometric Tessellation Pattern 4 No Background
Source GDJ
Seamless , tileable CC-0 texture. Created by my own, feel free to use wherever you want!
Source Linolafett
A beautiful dark padded pattern, like an old classic sofa.
Source Chris Baldie
Submitted as a black pattern, I made it light and a few steps more subtle.
Source Andy
The image depicts polka dot seamless pattern.
Source Yamachem
This one resembles a black concrete wall when is tiled. It should look great, at least with dark website themes.
Source V. Hartikainen
Derived from an image that was uploaded to Pixabay by starchim01
Source Firkin
A simple but elegant classic. Every collection needs one of these.
Source Christopher Burton
And some more testing, this time with Seamless Studio. It’s Robots FFS!
Source Seamless Studio
Inspired by a design found in 'Konstantinápolyi emlékeim', Miklos Chriszto, 1893.
Source Firkin