Not so subtle. These tileable wood patterns are very useful.
Source Elemis
Remixed from a design seen on Pixabay. The basic tile can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Prismatic Curved Diamond Pattern 5 No Background
Source GDJ
Prismatic Hexagonalism Pattern No Background
Source GDJ
Seamless , tileable CC-0 texture. Created by my own, feel free to use wherever you want!
Source Linolafett
A series of 5 patterns. That’s what the P stands for, if you didn’t guess it.
Source Dima Shiper
Prismatic Abstract Line Art Pattern Background 2
Source GDJ
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
To get the tile this is made up from select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Inspired by a pattern found in 'Sun Pictures of the Norfolk Broads', Ernest Suffling, 1892.
Source Firkin
A seamless texture of a rough concrete surface.
Source V. Hartikainen
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
This one takes you back to math class. Classic mathematic board underlay.
Source Josh Green
Floral patterns might not be the hottest thing right now, but you never know when you need it!
Source Lauren
He influenced us all. “Don’t be sad because it’s over. Smile because it happened.”
Source Atle Mo
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
CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
8 by 8 pixels, and just what the title says.
Source pixilated
Prismatic Snowflakes Pattern 3 No Background
Source GDJ