This seamless background image should look nice on websites. It has a dark blue gray texture with vertical stripes, it tiles seamlessly and, like all of the background images here, it's free. So, if you like it, take it!
Source V. Hartikainen
A comeback for you: the popular Escheresque, now in black.
Source Patten
I took the liberty of using Dmitry’s pattern and made a version without perforation.
Source Atle Mo
Derived from an image that was uploaded to Pixabay by mdmelo.
Source Firkin
A free background pattern with abstract green tiles.
Source V. Hartikainen
Old China with a modern twist, take two.
Source Adam Charlts
This one could be the shirt of a golf player. Angled lines in different thicknesses.
Source Olivier Pineda
A free seamless background image with abstract texture of green "curtain".
Source V. Hartikainen
Fix side and a seamless pattern formed from circles.
Source SliverKnight
Bit of a strange name on this one, but still nice. Tiny gray square things.
Source Carlos Valdez
The act or state of corrugating or of being corrugated, a wrinkle; fold; furrow; ridge.
Source Anna Litvinuk
Stefan is hard at work, this time with a funky pattern of squares.
Source Stefan Aleksić
Vertical lines with a bumpy, yet crisp, feel to it.
Source Raasa
Zero CC tileable wood texture, made by me procedurally in Neo Texture Edit.
Source Sojan Janso
Drawn in Paint.net and vectorised in Vector Magic.
Source Firkin
Nicely crafted paper pattern, although a bit on the large side (500x593px).
Source Blaq Annabiosis
ZeroCC tileable beechwood wood texture, generated in Neo Texture Edit by me. CC0
Source Sojan Janso
Utilising some flowers from Almeidah. To get the unit tile, select the rectangle in Inkscape and use shift-alt-i.
Source Firkin
Fix and cc0 to get the tile this is based on.
Source SliverKnight
Small dots with minor circles spread across to form a nice mosaic.
Source John Burks
Prismatic 3D Isometric Tessellation Pattern 6
Source GDJ
From a drawing in 'Art Embroidery', M.S. Lockwood and E. Glaister, 1878.
Source Firkin
I’m starting to think I have a concrete wall fetish.
Source Atle Mo
Just to prove my point, here is a slightly modified dark version.
Source Atle Mo
From a drawing in 'Line and form', Walter Crane, 1914.
Source Firkin