A seamless pattern created from a square tile. To get the tile, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Prismatic Curved Diamond Pattern 3 No Background
Source GDJ
A simple circle. That’s all it takes. This one is even transparent, for those who like that.
Source Saqib
An abstract texture of black metal pipes (seamless).
Source V. Hartikainen
The act or state of corrugating or of being corrugated, a wrinkle; fold; furrow; ridge.
Source Anna Litvinuk
Super simple but very nice indeed. Gray with vertical stripes.
Source Merrin Macleod
Derived from an image that was uploaded to Pixabay by darkmoon1968
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
Prismatic Curved Diamond Pattern 5 No Background
Source GDJ
Prismatic Chevrons Pattern 5 With Background
Source GDJ
A comeback for you: the popular Escheresque, now in black.
Source Patten
Dark, square, clean and tidy. What more can you ask for?
Source Jaromír Kavan
This one resembles a black concrete wall when is tiled. It should look great, at least with dark website themes.
Source V. Hartikainen
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
Prismatic Curved Diamond Pattern 4 No Background
Source GDJ
Super simple but very nice indeed. Gray with vertical stripes.
Source Merrin Macleod
Remixed from a drawing in 'Line and form', Walter Crane, 1914.
Source Firkin
Wild Oliva or Oliva Wilde? Darker than the others, sort of a medium dark pattern.
Source Badhon Ebrahim
Greyscale version of a pattern that came out of playing with the 'slinky' plug-in for Paint.net
Source Firkin
A light background pattern with diagonal stripes. Here's a simple light striped background for you.
Source V. Hartikainen