A repeating gloomy background image. This one consists of a pattern of black chains layered on top of a dark textured background.
Source V. Hartikainen
Sometimes simple really is what you need, and this could fit you well.
Source Factorio.us Collective
Prismatic Curved Diamond Pattern 8 No Background
Source GDJ
Zero CC tileable Laminate wood texture, photographed and made by me. CC0
Source Sojan Janso
Background formed from the iconic plastic construction bricks that gave me endless hours of fun when I was a lad.
Source Firkin
Turn your site into a dragon with this great scale pattern.
Source Alex Parker
Don’t look at this one too long if you’re high on something.
Source Luuk van Baars
The tile this is based on can be retrieved by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Prismatic Groovy Concentric Background 2 No Black
Source GDJ
I love cream! 50x50px and lovely in all the good ways.
Source Thomas Myrman
Zero CC tileable moss or lichen covered stone texture, edited from pixabay. CC0
Source Sojan Janso
Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
Background Wall, Art Abstract, white Well & CC0 texture.
Source Ractapopulous
One more updated pattern. Not really carbon fiber, but it’s the most popular pattern, so I’ll give you an extra choice.
Source Atle Mo
Prismatic Hexagonalist Pattern No Background
Source GDJ
ZeroCC tileable moss texture, photographed and made by me. CC0
Source Sojan Janso
Drawn in Paint.net and vectorised in Vector Magic.
Source Firkin
From a drawing in 'Artists and Arabs', Henry Blackburn, 1868.
Source Firkin
Remixed from a drawing in 'Очерки Русской Исторіи въ памятникахъ быта', Petr Polevoi, 1879.
Source Firkin
Prismatic Chevrons Pattern 5 With Background
Source GDJ
A seamless texture of an abstract wall colored in shades of light orange brown.
Source V. Hartikainen
And some more testing, this time with Seamless Studio. It’s Robots FFS!
Source Seamless Studio
This background has abstract texture with some similarities to wood.
Source V. Hartikainen
With a name this awesome, how can I go wrong?
Source Nikolay Boltachev