A repeating background of thick textured paper. Actually, it turned out to look like something between a paper and fabric.
Source V. Hartikainen
After 1 comes 2, same but different. You get the idea.
Source Hendrik Lammers
Simple wide squares with a small indent. Fits all.
Source Petr Šulc.
Bright Multicolored Floral Background by Karen Arnold from PDP.
Source GDJ
Here's a seamless brown cork board background texture. Feel free to download or reshare if you like.
Source V. Hartikainen
Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
Prismatic Polka Dots 3 No Background
Source GDJ
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
Prismatic Abstract Geometric Background 3
Source GDJ
Drawn in Paint.net using the kaleidoscope plug-in and vectorised.
Source Firkin
This is a remix of "flower seamless pattern".I rotated the original image by 90 degrees.This is a seamless pattern of flowers.These horizontal wavy lines are one of Edo patterns which is called "tatewaku or tachiwaku or 立湧" that represents uprising steam or vapor.
Source Yamachem
The tile this is based on can be had by selecting the rectangle in Inkscape and using 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 seamless pattern with green and yellow diagonal lines on top of a white dotted background.
Source V. Hartikainen
I scanned a paper coffee cup. You know, in case you need it.
Source Atle Mo
Seamless pattern made from a tile that can be obtained in Inkscape by selecting the rectangle and using shift+alt+i.
Source Firkin
A comeback for you: the popular Escheresque, now in black.
Source Patten
This was formed by distorting an image of a background on Pixabay.
Source Firkin
ZeroCC tileable stone texture, edited from pixabay. CC0
Source Sojan Janso
Coming in at 666x666px, this is an evil big pattern, but nice and soft at the same time.
Source Atle Mo
Remixed from a drawing in 'Sun Pictures of the Norfolk Broads', Ernest Suffling, 1892. The tile this is based on can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin
Fix and cc0 to get the tile this is based on.
Source SliverKnight
You know I’m a sucker for these. Well-crafted paper pattern.
Source Mihaela Hinayon