Nice and simple crossed lines in dark gray tones.
Source Stefan Aleksić
Geometric triangles seem to be quite hot these days.
Source Pixeden
Just to prove my point, here is a slightly modified dark version.
Source Atle Mo
Horizontal and vertical lines on a light gray background.
Source Adam Anlauf
This one is amazing, truly original. Go use it!
Source Viahorizon
This is so subtle I hope you can see it! Tweak at will.
Source Alexandre Naud
Seamless pattern formed from a tile that can be extracted by selecting the rectangle in Inkscape and using shift-alt-i.
Source Firkin
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 raster on Pixabay, that was uploaded by ArtsyBee.
Source Firkin
Actually remixed from a pattern on Pixabay. But then noticed a very similar one on Openclipart.org uploaded by btj51q2.
Source Firkin
A seamless pattern based on a square tile that can be retrieved in Inkscape by selecting the rectangle and using shift-alt-i.
Source Firkin
Turn your site into a dragon with this great scale pattern.
Source Alex Parker
Simple wide squares with a small indent. Fits all.
Source Petr Šulc.
I skipped number 3, because it wasn’t all that great. Sorry.
Source Dima Shiper
From a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
A seamless texture of a rough concrete surface.
Source V. Hartikainen
Prismatic Polyskelion Pattern No Background
Source GDJ
This one needs to be used in small areas; you can see it repeat.
Source Luca
You may use it as is, or modify it as you like.
Source V. Hartikainen