Dare I call this a «flat pattern»? Probably not.
Source Dax Kieran
Prismatic Groovy Concentric Background 4 No Black
Source GDJ
Prismatic Hexagonalist Pattern No Background
Source GDJ
A seamless pattern formed from a sports car on clker.com. To get the tile, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Prismatic 3D Isometric Tessellation Pattern 6
Source GDJ
This was submitted in a beige color, hence the name. Now it’s a gray paper pattern.
Source Konstantin Ivanov
This light blue background pattern is quite pleasing to the eye, it consists of a tiny rough grid pattern, which is seamless by design. That's it, if you like the color, you can use this seamless pattern in a web design without making any further modifications to it.
Source V. Hartikainen
Dark pattern with some nice diagonal stitched lines crossing over.
Source Ashton
To get the tile this is based on select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Prismatic Hexagonalism Pattern No Background
Source GDJ
Prismatic Groovy Concentric Background 3 No Black
Source GDJ
Got some felt in my mailbox today, so I scanned it for you to use.
Source Atle Mo
A very dark asfalt pattern based off of a photo taken with my iPhone.
Source Atle Mo
Did some testing with Repper Pro tonight, and this gray mid-tone pattern came out.
Source Atle Mo
Alternative colour scheme. Not a pattern for fabrics, but one produced from a jpg of a stack of fabric items that was posted on Pixabay. The tile that this is based on can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
A green background pattern with warped vertical stripes and a grunge look.
Source V. Hartikainen
Prismatic Hypnotic Pattern No Background
Source GDJ
To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
A background pattern inspired by designs seen in 'Burghley. The Life of William Cecil', William Charlton, 1857.
Source Firkin
A free web background image with a seamless concrete-like texture and an Indian-red color.
Source V. Hartikainen
From a drawing in 'Line and form', Walter Crane, 1914.
Source Firkin