Here's a subtle marble-like background for use on websites.
Source V. Hartikainen
Prismatic Chevrons Pattern 5 With Background
Source GDJ
CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
Pattern #100! A black classic knit-looking pattern.
Source Factorio.us Collective
Seamless Prismatic Pythagorean Line Art Pattern No Background. A seamless pattern that includes the original tile (go to Objects / Pattern / Pattern To Objects in Inkscape's menu to extract it).
Source GDJ
Prismatic Abstract Line Art Pattern Background
Source GDJ
Same as gray sand but lighter. A sandy pattern with small light dots, and some angled strokes.
Source Atle Mo
Remixed from a drawing in 'Chambéry à la fin du XIVe siècle', Timoleon Chapperon, 1863.
Source Firkin
ZeroCC tileable stone texture, edited from pixabay. CC0
Source Sojan Janso
It’s an egg, in the form of a pattern. This really is 2012.
Source Paul Phönixweiß
This could be a hippy vintage wallpaper.
Source Tileable Patterns
Vector version of a JPG that was uploaded to Pixabay by theasad121
Source Firkin
Prismatic Geometric Tessellation Pattern 2 No Background
Source GDJ
This is a grid, only it’s noisy. You know. Reminds you of those printed grids you draw on.
Source Vectorpile
Prismatic Abstract Geometric Background 5
Source GDJ
This is indeed a bit strange, but here’s to the crazy ones!
Source Christopher Buecheler
From a drawing in 'Line and form', Walter Crane, 1914.
Source Firkin
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
A seamless pattern formed from a modified version of rwwgub's tile. To get the tile, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
From a drawing in 'Cowdray: the history of a great English House', Julia Roundell, 1884.
Source Firkin
From a drawing in 'A Rolling Stone. A tale of wrongs and revenge', John Hartley, 1878.
Source Firkin