Seamless pattern formed from a tile that can be had by selecting the rectangle in Inkscape and using shift-alt-i.
Source Firkin
A comeback for you: the popular Escheresque, now in black.
Source Patten
Here's a camo print with more tan and less green, such as might be used in a desert scenario. This is tileable, so it can be used as a wallpaper or background.
Source Eady
Tiny circle waves, almost like the ocean.
Source Sagive
ZeroCC tileable beechwood wood texture, generated in Neo Texture Edit by me. CC0
Source Sojan Janso
Prismatic Groovy Concentric Background 3 No Black
Source GDJ
Prismatic Abstract Geometric Background 5 No Black
Source GDJ
There are many carbon patterns, but this one is tiny.
Source Designova
Recreated from a pattern found in 'Az Osztrák-Magyar Monarchia irásban és képben', 1882. To get the tile this is based on select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
A seamless pattern from a tile made from a jpg on Pixabay. To get the tile select the rectangle in Inkscape and use shift-alt-i.
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
Dare I call this a «flat pattern»? Probably not.
Source Dax Kieran
From a drawing in 'Art Embroidery', M.S. Lockwood and E. Glaister, 1878.
Source Firkin
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
New paper pattern with a slightly organic feel to it, using some thin threads.
Source Atle Mo
Have you wondered about how it feels to be buried alive? Here is the pattern for it.
Source Hendrik Lammers
Seamless pattern formed from a tile that can be had by selecting the rectangle in Inkscape and using shift-alt-i.
Source Firkin
One of the few full-color patterns here, but this one was just too good to pass up.
Source Alexey Usoltsev