This could be a hippy vintage wallpaper.
Source Tileable Patterns
Looks a bit like concrete with subtle specks spread around the pattern.
Source Mladjan Antic
Here's a subtle marble-like background for use on websites.
Source V. Hartikainen
Nicely crafted paper pattern, although a bit on the large side (500x593px).
Source Blaq Annabiosis
To get the tile this is based on, select the rectangle in Inkscape and use shift-alt-i.
Source Firkin
Embossed lines and squares with subtle highlights.
Source Alex Parker
A series of 5 patterns. That’s what the P stands for, if you didn’t guess it.
Source Dima Shiper
A textured blue background pattern with vertical stripes.
Source V. Hartikainen
Black And White Floral Pattern Background Inverse
Source GDJ
Green Background Pattern
Source V. Hartikainen
Prismatic Geometric Pattern Background 2
Source GDJ
Because I love dark patterns, here is Brushed Alum in a dark coating.
Source Tim Ward
From a drawing of the coat of arms of the Ottoman Empire on Wikimedia.
Source Firkin
Remixed from a PNG that was uploaded to Pixabay by gingertea
Source Firkin
Turn your site into a dragon with this great scale pattern.
Source Alex Parker
A background pattern inspired by designs seen in 'Burghley. The Life of William Cecil', William Charlton, 1857.
Source Firkin
Light square grid pattern, great for a “DIY projects” sort of website, maybe?
Source Rafael Almeida
Prismatic Abstract Geometric Background 4
Source GDJ
A re-make of the Gradient Squares pattern.
Source Dimitar Karaytchev
Nothing like a clean set of bed sheets, huh?
Source Badhon Ebrahim
A colourful background drawn originally in paint.net and vectorised in Vector Magic.
Source Firkin
Gold Triangular Seamless Pattern No Background
Source GDJ
Run a restaurant blog? Here you go. Done.
Source Andrijana Jarnjak