Simple wide squares with a small indent. Fits all.
Source Petr Šulc.
A seamless pattern the starting point for which was a 'rainbow twist' texture in Paint.net.
Source Firkin
A seamless paper background colored in pale yellow.
Source V. Hartikainen
Original minus the background
Source Firkin
Another fairly simple design drawn in Paint.net and vectorized in Inkscape.
Source Firkin
Horizontal and vertical lines on a light gray background.
Source Adam Anlauf
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
This is indeed a bit strange, but here’s to the crazy ones!
Source Christopher Buecheler
Dark pattern with some nice diagonal stitched lines crossing over.
Source Ashton
Prismatic Abstract Geometric Background derived from an image on Pixabay.
Source GDJ
Like the name suggests, this background image consists of a pattern of dark bricks. It may be an option for you, if you are looking for something that looks like a brick wall for use as a background on web pages. It's not a masterpiece, but looks pretty nice when is tiled.
Source V. Hartikainen
A pale yellow background pattern with vertical stripes. The stripes are partially faded. I think this background image turned out pretty well, especially those faded stripes.
Source V. Hartikainen
A dark one with geometric shapes and dotted lines.
Source Mohawk Studios
Could be paper, could be a Polaroid frame – up to you!
Source Chaos
Classic vertical lines, in all its subtlety.
Source Cody L
Abstract Stars Geometric Pattern Prismatic No Background
Source GDJ
Prismatic Groovy Concentric Background 3 No Black
Source GDJ
One of the few full-color patterns here, but this one was just too good to pass up.
Source Alexey Usoltsev
Light square grid pattern, great for a “DIY projects” sort of website, maybe?
Source Rafael Almeida
From a drawing in 'Artists and Arabs', Henry Blackburn, 1868.
Source Firkin
A seamless pattern formed from miutopia's cakes on a tablecloth.
Source Firkin
Luxury pattern, looking like it came right out of Paris.
Source Daniel Beaton