Here's a repeatable texture that resembles a light green concrete wall or something similar.
Source V. Hartikainen
Prismatic Curved Diamond Pattern 3 No Background
Source GDJ
The name alone is awesome, but so is this sweet dark pattern.
Source Federica Pelzel
Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
This one is amazing, truly original. Go use it!
Source Viahorizon
From a drawing in 'Codogno e il suo territorio nella cronaca e nella storia'', Gio and Giarella Cairo, 1897.
Source Firkin
Sounds French. Some 3D square diagonals, that’s all you need to know.
Source Graphiste
Prismatic Rounded Squares Grid 4 No Background
Source GDJ
From a drawing in 'Studies for Stories', Jean Ingelow, 1864.
Source Firkin
A seamless pattern drawn originally in Paint.net by distorting a slice of background pattern 116 and copying the resulting triangle numerous times.
Source Firkin
Scanned some rice paper and tiled it up for you. Enjoy.
Source Atle Mo
The original enhanced with one of Inkscapes's filters.
Source Firkin
Prismatic Polka Dots Mark II No Background
Source GDJ
A free black metallic background pattern. Here's a new pattern I made that looks metallic.
Source V. Hartikainen
A yellow tiled background... Blurriness, bokeh effect and rectangles pattern in one mix.
Source V. Hartikainen
Very simple, very blu(e). Subtle and nice.
Source Seb Jachec
A seamless textured paper for backgrounds. Colored in pale orange hues.
Source V. Hartikainen
Remixed from a JPG that was uploaded to Pixabay by Osckar
Source Firkin
A seamless texture of black leather. I think it will look best when used in headers, footers or sidebars.
Source V. Hartikainen
Fix and cc0 to get the tile this is based on.
Source SliverKnight
Remixed from a drawing in 'Sun Pictures of the Norfolk Broads', Ernest Suffling, 1892.
Source Firkin
Prismatic Groovy Concentric Background 2 No Black
Source GDJ
An alternative colour scheme to the original seamless pattern.
Source Firkin