Seamless pattern formed from a tile that can be extracted by selecting the rectangle in Inkscape and using shift-alt-i.
Source Firkin
Abstract Arbitrary Geometric Background derived from an image on Pixabay.
Source GDJ
CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
By popular request, an outline version of the pentagon pattern.
Source Atle Mo
A seamless pattern the unit cell for which can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin
Kaleidoscope Prismatic Abstract No Background
Source GDJ
Remixed from an image that was uploaded to Pixabay by Pixeline
Source Firkin
Dare I call this a «flat pattern»? Probably not.
Source Dax Kieran
Abstract Tiled Background Extended 10
Source GDJ
Adapted heavily from a JPG that was uploaded to Pixabay by Viscious-Speed.
Source Firkin
From a drawing in 'From Snowdon to the Sea. Striking stories of North and South Wales', Marie Trevelyan, 1895.
Source Firkin
A beautiful dark padded pattern, like an old classic sofa.
Source Chris Baldie
A seamless pattern formed from background pattern 102
Source Firkin
A background pattern with wavy green vertical stripes. This one has green stripes on a white background. Download if you like it.
Source V. Hartikainen
Remixed from a JPG that was uploaded to Pixabay by theasad121
Source Firkin
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
Nicely executed tiling for an interesting pattern.
Source Ignasi Àvila Padró
This is so subtle I hope you can see it! Tweak at will.
Source Alexandre Naud
More tactile goodness. This time in the form of some rough cloth.
Source Bartosz Kaszubowski
Pixel by pixel, sharp and clean. Very light pattern with clear lines.
Source M.Ashok
You can never get enough of these tiny pixel patterns with sharp lines.
Source Designova