A huge one at 800x600px. Made from a photo I took going home after work.
Source Atle Mo
A background formed from an image of an old tile on the New York Metropolitan Museum of Art website. To get the base tile, select the rectangle in Inkscape and use shift-alt-i.
Source Firkin
A seamless pattern formed from background pattern 102
Source Firkin
I skipped number 3, because it wasn’t all that great. Sorry.
Source Dima Shiper
Remixed from a JPG that was uploaded to Pixabay by theasad121
Source Firkin
Formed from a tile based on a drawing from 'Viaggi d'un artista nell'America Meridionale', Guido Boggiani, 1895.
Source Firkin
Basket Fibers, Basket Texture, Braid Background style CC0 texture.
Source 1A-Photoshop
Black brick wall pattern. Brick your site up!
Source Alex Parker
One more in the line of patterns inspired by Japanese/Asian styles. Smooth.
Source Kim Ruddock
This light background pattern has a texture of "frozen" surface with diagonal stripes. Here's an yet another addition to the collection of free website backgrounds.
Source V. Hartikainen
Light square grid pattern, great for a “DIY projects” sort of website, maybe?
Source Rafael Almeida
Remixed from an image that was uploaded to Pixabay by Pixeline
Source Firkin
If you want png files of this u can download them here : viscious-speed.deviantart.com/gallery/27635117
Source Viscious-Speed
Remixed from a design seen in 'Burghley. The Life of William Cecil', William Charlton, 1857.
Source Firkin
Green Web Background, Seamless tile.
Source V. Hartikainen
A monochrome pattern from a tile that can be had by selecting the rectangle in Inkscaope and using shift+alt+i
Source Firkin
Based on an image that was uploaded to Pixabay by devanath
Source Firkin
From a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Prismatic Geometric Tessellation Pattern 4 No Background
Source GDJ
Prismatic Abstract Geometric Background 5 No Black
Source GDJ
The image depicts a Japanese Edo pattern called "kanoko or 鹿の子" meaning "fawn" which has a fur with small white spots.
Source Yamachem
Smooth Polaroid pattern with a light blue tint.
Source Daniel Beaton