By popular request, an outline version of the pentagon pattern.
Source Atle Mo
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
A comeback for you: the popular Escheresque, now in black.
Source Patten
Prismatic Groovy Concentric Background No Black
Source GDJ
Dare I call this a «flat pattern»? Probably not.
Source Dax Kieran
A seamless pattern formed from miutopia mug remixes on a tablecloth.
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
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
To get the tile this is based on select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
To get the tile this is based on select the rectangle in Inkscape and use shift-alt-i
Source Firkin
A repeating background of beige (or is it more vanilla yellow) textured stripes. One more background with stripes.
Source V. Hartikainen
Like the name suggests, this background image consists of a pattern of dark bricks. It may be an option for you, if you are looking for something that looks like a brick wall for use as a background on web pages. It's not a masterpiece, but looks pretty nice when is tiled.
Source V. Hartikainen
Prismatic Curved Diamond Pattern 5 No Background
Source GDJ
A bit of scratched up grayness. Always good.
Source Dmitry
Prismatic Groovy Concentric Background 5 No Black
Source GDJ
Lovely pattern with splattered vintage speckles.
Source David Pomfret
After 1 comes 2, same but different. You get the idea.
Source Hendrik Lammers
8 by 8 pixels, and just what the title says.
Source pixilated
Subtle scratches on a light gray background.
Source Andrey Ovcharov
Formed by distorting the inside front cover of 'Diversæ insectarum volatilium : icones ad vivum accuratissmè depictæ per celeberrimum pictorem', Jacob Hoefnagel, 1630.
Source Firkin