Adapted heavily from a JPG that was uploaded to Pixabay by Viscious-Speed.
Source Firkin
Dare I call this a «flat pattern»? Probably not.
Source Dax Kieran
The unit cell for this seamless pattern can be had in Inkscape by selecting the rectangle and using shift+alt+i.
Source Firkin
A grid of squares with green colours. Since the colours are randomly distributed it is automatically seamless.
Source Firkin
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 seamless pattern formed from a square tile. The tile can be retrieved by selecting the rectangle in Inkscape and using shift-alt-I.
Source Firkin
Prismatic Rounded Squares Grid 3 No Background
Source GDJ
Used in small doses, this could be a nice subtle pattern. Used on a large surface, it’s dirty!
Source Paul Reulat
And some more testing, this time with Seamless Studio. It’s Robots FFS!
Source Seamless Studio
A seamless pattern with wide vertical stripes colored in pale yellow.
Source V. Hartikainen
This pattern comes in orange, and it looks as if it is "made of glass".
Source V. Hartikainen
From a drawing in 'Line and form', Walter Crane, 1914.
Source Firkin
A background pattern with blue on white vertical stripes.
Source V. Hartikainen
Wild Oliva or Oliva Wilde? Darker than the others, sort of a medium dark pattern.
Source Badhon Ebrahim
Pass parameters to the URL or edit the source code variables to configure the graph paper for the division desired.
Source JayNick
Zero CC asphalt, pavement, texture, photographed and made by me. CC0 WARNING I FOUND A SEAM ON THIS TEXTURE
Source Sojan Janso
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
Inspired by this, I came up with this pattern. Madness!
Source Atle Mo
This pack of filters can help you adding a blocky overlay to objects. May come handy at drawing blocks of stone.
Source Lazur URH
ZeroCC tileable stone texture, edited from pixabay. CC0
Source Sojan Janso
Prismatic Polka Dots 3 No Background
Source GDJ
An abstract texture of black metal pipes (seamless).
Source V. Hartikainen