Sometimes simple really is what you need, and this could fit you well.
Source Factorio.us Collective
Derived from an image that was uploaded to Pixabay by Darkmoon1968
Source Firkin
Inspired by a pattern found in 'Sun Pictures of the Norfolk Broads', Ernest Suffling, 1892.
Source Firkin
From a drawing of the coat of arms of the Ottoman Empire on Wikimedia.
Source Firkin
As the original image 's page size is too large for its image size, I remixed it.
Source Yamachem
I asked Gjermund if he could make a pattern for us – result!
Source Gjermund Gustavsen
Sharp pixel pattern looking like some sort of fabric.
Source Dmitry
From a drawing in 'Art Embroidery', M.S. Lockwood and E. Glaister, 1878.
Source Firkin
This is indeed a bit strange, but here’s to the crazy ones!
Source Christopher Buecheler
A gray background pattern with a texture of textile. Suits perfectly for web design.
Source V. Hartikainen
Remixed from a drawing in 'Line and form", Walter Crane, 1914.
Source Firkin
A seamless pattern that includes the original tile (go to Objects / Pattern / Pattern To Objects in Inkscape's menu to extract it).
Source GDJ
Dare I call this a «flat pattern»? Probably not.
Source Dax Kieran
This one could be the shirt of a golf player. Angled lines in different thicknesses.
Source Olivier Pineda
A seamless pattern the starting point for which was a 'rainbow twist' texture in Paint.net.
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
Drawn in Paint.net and vectorised in Vector Magic
Source Firkin
Heavy depth and shadows here, but might work well on some mobile apps.
Source Damian Rivas
A playful triangle pattern with different shades of gray.
Source Dimitrie Hoekstra
This is so subtle I hope you can see it! Tweak at will.
Source Alexandre Naud
Abstract Tiled Background Extended 11
Source GDJ