Dark squares with some virus-looking dots in the grid.
Source Hugo Loning
To get the tile this is based on select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
One of the few full-color patterns here, but this one was just too good to pass up.
Source Alexey Usoltsev
From a drawing in 'Les Chroniqueurs de l'Histoire de France depuis les origines jusqu'au XVIe siècle', Henriette Witt, 1884.
Source Firkin
Dark, square, clean and tidy. What more can you ask for?
Source Jaromír Kavan
Don’t look at this one too long if you’re high on something.
Source Luuk van Baars
Prismatic Abstract Geometric Background 2
Source GDJ
Remixed from a drawing in 'Works. Popular edition', John Ruskin, 1886.
Source Firkin
One more in the line of patterns inspired by Japanese/Asian styles. Smooth.
Source Kim Ruddock
It almost looks a bit blurry, but then again, so are fishes.
Source Petr Šulc
Zero CC tileable yellow craft paper; scanned and made by me. CC0
Source Sojan Janso
Cubes as far as your eyes can see. You know, because they tile.
Source Jan Meeus
Seamless Prismatic Geometric Pattern With Background
Source GDJ
Background Wall, Art Abstract, Blue Well & CC0 texture.
Source Ractapopulous
Textured Red Brown Plastic, Free Background Pattern. Although there's already enough plastic in our lives, let's bring it to the web too.)
Source V. Hartikainen