Here's a subtle marble-like background for use on websites.
Source V. Hartikainen
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
A free seamless texture of reptile skin colored in a dark brown color. As always, you may use it as a repeated background image in your web design works, or for any other purposes.
Source V. Hartikainen
Design drawn in Paint.net, vectorised using Vector Magic and finished in Inkscape.
Source Firkin
Design drawn in Paint.net, vectorised using Vector Magic and finished in Inkscape.
Source Firkin
Wild Oliva or Oliva Wilde? Darker than the others, sort of a medium dark pattern.
Source Badhon Ebrahim
A beautiful dark wood pattern, superbly tiled.
Source Omar Alvarado
Inspired by this, I came up with this pattern. Madness!
Source Atle Mo
Derived from an image that was uploaded to Pixabay by mdmelo.
Source Firkin
Lovely light gray floral motif with some subtle shades.
Source GraphicsWall
Smooth Polaroid pattern with a light blue tint.
Source Daniel Beaton
Drawn in Paint.net and vectorised in Vector Magic.
Source Firkin
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
No, not the band but the pattern. Simple squares in gray tones, of course.
Source Atle Mo
This is a hot one. Small, sharp and unique.
Source GraphicsWall
A series of 5 patterns. That’s what the P stands for, if you didn’t guess it.
Source Dima Shiper
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
A seamless background drawn in Paint.net and vectorised with Vector Magic. The starting point was a photograph of drinking straws from Pixabay.
Source Firkin
Seamless Light Background Texture.
Source V. Hartikainen
The green fibers pattern will work very well in grayscale as well.
Source Matteo Di Capua
Prismatic Polka Dots Mark II No Background
Source GDJ
Nicely executed tiling for an interesting pattern.
Source Ignasi Àvila Padró
Light square grid pattern, great for a “DIY projects” sort of website, maybe?
Source Rafael Almeida
Colour version of the original seamless pattern.
Source Firkin
A free seamless background image with a texture of dark red "canvas". It should look very nice on web sites.
Source V. Hartikainen
Sort of like the Photoshop transparent background, but better!
Source Alex Parker