A light background pattern with diagonal stripes. Here's a simple light striped background for you.
Source V. Hartikainen
A grid of squares with green colours. Since the colours are randomly distributed it is automatically seamless.
Source Firkin
Tiny circle waves, almost like the ocean.
Source Sagive
Background Wall, Art Abstract, Block Well & CC0 texture.
Source Ractapopulous
Clover with background for St. Patrick's Day. Add to a card with a doily, ribbon, a leprechaun or other embellishments.
Source BAJ
Black And White Floral Pattern Background from PDP.
Source GDJ
A slightly more textured pattern, medium gray. A bit like a potato sack?
Source Bilal Ketab
Detailed but still subtle and quite original. Lovely gray shades.
Source Kim Ruddock
Tiny, tiny 3D cubes. Reminds me of the good old pattern from k10k.
Source Etienne Rallion
Prismatic Floral Background No Black
Source GDJ
Black & white version of a pattern that came out of playing with the 'light rays' plug-in for Paint.net
Source Firkin
emixed from a JPG that was uploaded to Pixabay by Kyotime
Source Firkin
This one resembles a black concrete wall when is tiled. It should look great, at least with dark website themes.
Source V. Hartikainen
An abstract pale yellow paper-like background with stains colored in yellow and green.
Source V. Hartikainen
Redrawn based on a drawing in 'По Сѣверо-Западу Россіи' Konstantin Sluchevsky, 1897.
Source Firkin
As far as fabric patterns goes, this is quite crisp.
Source Heliodor Jalba