Seamless pattern the tile for which can be had by using shift-alt-I on the selected rectangle in Inkscape.
Source Firkin
Hexagonal dark 3D pattern. What more can you ask for?
Source Norbert Levajsics
A black tile-able background with paper-like texture.
Source V. Hartikainen
Black brick wall pattern. Brick your site up!
Source Alex Parker
Prismatic Geometric Pattern Background
Source GDJ
A seamless pattern from a tile made from a jpg on Pixabay. To get the tile select the rectangle in Inkscape and use shift-alt-i.
Source Firkin
This background pattern looks like bamboo to me. Feel free to download it for your website (for your blog perhaps?).
Source V. Hartikainen
Kaleidoscope Prismatic Abstract No Background
Source GDJ
One more brick pattern. A bit more depth to this one.
Source Benjamin Ward
Design drawn in Paint.net, vectorised using Vector Magic and finished in Inkscape.
Source Firkin
From a drawing in 'La Principauté de Liège et les Pays-Bas au XVIe siècle', Société des Bibliophiles Liégeois ,1887.
Source Firkin
Crossing lines with a subtle emboss effect on a dark background.
Source Stefan Aleksić
A seamless pattern formed from a square tile. To get the tile, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
I guess this one is inspired by an office. A dark office.
Source Andrés Rigo.
Remixed from a drawing in 'A Child of the Age', Francis Adams, 1894.
Source Firkin
From a drawing in 'Sun Pictures of the Norfolk Broads', Ernest Suffling, 1892.
Source Firkin
Submitted by DomainsInfo – wtf, right? But hey, a free pattern.
Source DomainsInfo
This is so subtle: We’re talking 1% opacity. Get your squint on!
Source Atle Mo
Not a flat you live inside, like in the UK – but a flat piece of cardboard.
Source Appleshadow
New paper pattern with a slightly organic feel to it, using some thin threads.
Source Atle Mo
Remixed from a drawing in 'Line and form', Walter Crane, 1914.
Source Firkin
This one is so simple, yet so good. And you know it. Has to be in the collection.
Source Gluszczenko