A good starting point for a cardboard pattern. This would work well in a variety of colors.
Source Atle Mo
Prismatic Geometric Pattern Background 2 No Black
Source GDJ
Nothing like a clean set of bed sheets, huh?
Source Badhon Ebrahim
CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
From a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Seamless pattern formed from a tile that can be extracted by selecting the rectangle in Inkscape and using shift-alt-i.
Source Firkin
Sweet and subtle white plaster with hints of noise and grunge.
Source Phil Maurer
This one takes you back to math class. Classic mathematic board underlay.
Source Josh Green
Inspired by a 1930s wallpaper pattern I saw on TV.
Source Firkin
Number 4 in a series of 5 beautiful patterns. Can be found in colors on the submitter’s website.
Source Janos Koos
An abstract Background pattern of purple twisty patterns.
Source TikiGiki
Almost like little fish shells, or dragon skin.
Source Graphiste
Heavily remixed from a drawing that was uploaded to Pixabay by ractapopulous
Source Firkin
A seamless pattern from a tile made from a jpg on Pixabay. To get the tile select the rectangle in Inkscape and use shift-alt-i.
Source Firkin
From a drawing in 'Sun Pictures of the Norfolk Broads', Ernest Suffling, 1892.
Source Firkin
This seamless background image should look nice on websites. It has a dark blue gray texture with vertical stripes, it tiles seamlessly and, like all of the background images here, it's free. So, if you like it, take it!
Source V. Hartikainen
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
The act or state of corrugating or of being corrugated, a wrinkle; fold; furrow; ridge.
Source Anna Litvinuk
Drawn in Paint.net using the kaleidoscope plug-in and vectorised.
Source Firkin
Dark and hard, just the way we like it. Embossed triangles makes a nice pattern.
Source Ivan Ginev
A seamless pattern recreated from an image on Pixabay. It is reminiscent of parquet flooring and is formed from a square tile, which can be recovered in Inkscape by selecting the ungrouped rectangle and using shift-alt-I together.
Source Firkin