Based on several public domain drawings on Wikimedia Commons. This was formed from a rectangular tile. The tile can be accessed in Inkscape by selecting the rectangle and using shift-alt-i.
Source Firkin
A dark pattern made out of 3×3 circles and a 1px shadow. This works well as a carbon texture or background.
Source Atle Mo
So tiny, just 7 by 7 pixels – but still so sexy. Ah yes.
Source Dmitriy Prodchenko
This was submitted in a beige color, hence the name. Now it’s a gray paper pattern.
Source Konstantin Ivanov
This pattern comes in orange, and it looks as if it is "made of glass".
Source V. Hartikainen
From an image on opengameart.org shared by rubberduck.
Source Firkin
Dark, square, clean and tidy. What more can you ask for?
Source Jaromír Kavan
A seamless pattern based on a rectangular tile that can be retrieved in Inkscape by selecting the rectangle and using shift-alt-i.
Source Firkin
A bit strange this one, but nice at the same time.
Source Diogo Silva
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
Prismatic Groovy Concentric Background 3 No Black
Source GDJ
Colour version of the original seamless pattern.
Source Firkin
Dare I call this a «flat pattern»? Probably not.
Source Dax Kieran
From a drawing in 'Chambéry à la fin du XIVe siècle', Timoleon Chapperon, 1863.
Source Firkin
A nice and simple white rotated tile pattern.
Source Another One
A free seamless background image with abstract texture of green "curtain".
Source V. Hartikainen
Remixed from a drawing in 'Prehistoric Man: researches into the origin of civilisation in the old and the new world', Daniel Wilson, 1876.
Source Firkin
Zero CC tileable ground cracked, crackled, texture, made by me.
Source Sojan Janso
Subtle scratches on a light gray background.
Source Andrey Ovcharov
Submitted in a cream color, but you know how I like it.
Source Devin Holmes