It’s a hole, in a pattern. On your website. Dig it!
Source Josh Green
Tiny, tiny 3D cubes. Reminds me of the good old pattern from k10k.
Source Etienne Rallion
Inspired by a pattern seen on a public domain image of a very old tile. To get the unit cell, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Remixed from a design seen in 'Burghley. The Life of William Cecil', William Charlton, 1857. The tile this is based on can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
Looks like a technical drawing board: small squares forming a nice grid.
Source We Are Pixel8
Crossing lines with a subtle emboss effect on a dark background.
Source Stefan Aleksić
From a drawing in 'Gately's World's Progress', Charles Beale, 1886.
Source Firkin
Prismatic Curved Diamond Pattern 3 No Background
Source GDJ
This one takes you back to math class. Classic mathematic board underlay.
Source Josh Green
Light square grid pattern, great for a “DIY projects” sort of website, maybe?
Source Rafael Almeida
A seamless background pattern with impressed gray dots.
Source V. Hartikainen