You may use it as is, or modify it as you like.
Source V. Hartikainen
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
Retro Circles Background 8 No Black
Source GDJ
This one could be the shirt of a golf player. Angled lines in different thicknesses.
Source Olivier Pineda
Greyscale version of a pattern that came out of playing with the 'light rays' plug-in for Paint.net
Source Firkin
Based on an image that was uploaded to Pixabay by devanath
Source Firkin
A background pattern inspired by designs seen in 'Burghley. The Life of William Cecil', William Charlton, 1857.
Source Firkin
Prismatic Geometric Tessellation Pattern 3 No Background
Source GDJ
There are many carbon patterns, but this one is tiny.
Source Designova
This is so subtle: We’re talking 1% opacity. Get your squint on!
Source Atle Mo
From a drawing in 'Resa i Afrika, genom Angola, Ovampo och Damaraland', P. Moller, 1899.
Source Firkin
Nice and simple crossed lines in dark gray tones.
Source Stefan Aleksić
A pattern formed from repeated instances of corner decoration 8. To get the basic tile select the rectangle in Inkscape and use shift+alt+i
Source Firkin
From a drawing in 'Les Chroniqueurs de l'Histoire de France depuis les origines jusqu'au XVIe siècle', Henriette Witt, 1884.
Source Firkin
I skipped number 3, because it wasn’t all that great. Sorry.
Source Dima Shiper
Number 3 in a series of 5 beautiful patterns. Can be found in colors on the submitter’s website.
Source Janos Koos
You know I’m a sucker for these. Well-crafted paper pattern.
Source Mihaela Hinayon
A grid of squares with green colours. Since the colours are randomly distributed it is automatically seamless.
Source Firkin
The classic 45-degree diagonal line pattern, done right.
Source Jorick van Hees
Prismatic Curved Diamond Pattern No Background
Source GDJ
The image depicts a seamless pattern of a snow crystal.I referred to a book called ”sekka-zusetsu” or "雪華図説" which means an illustrated explanation about snow crystals.This book was published in 1832 (天保3年) or Edo period.For more about "雪華図説",see here:dl.ndl.go.jp/info:ndljp/pid/2536975
Source Yamachem
Not so subtle. These tileable wood patterns are very useful.
Source Elemis
A repeating background for websites with a texture of black groove stripes.
Source V. Hartikainen