A grid of squares with green colours. Since the colours are randomly distributed it is automatically seamless.
Source Firkin
Could be paper, could be a Polaroid frame – up to you!
Source Chaos
Just to prove my point, here is a slightly modified dark version.
Source Atle Mo
Remixed from a drawing in 'Maidenhood; or, the Verge of the Stream', Laura Jewry, 1876.
Source Firkin
Zero CC tileable yellow craft paper; scanned and made by me. CC0
Source Sojan Janso
A seamless pattern the starting point for which was a 'rainbow twist' texture in Paint.net.
Source Firkin
Love the style on this one, very fresh. Diagonal diamond pattern. Get it?
Source INS
The original enhanced with one of Inkscapes's filters.
Source Firkin
A beautiful dark padded pattern, like an old classic sofa.
Source Chris Baldie
From a drawing in 'A Life Interest', Mrs Alexander, 1888.
Source Firkin
A car pattern?! Can it be subtle? I say yes!
Source Radosław Rzepecki
This pack of filters can help you adding a blocky overlay to objects. May come handy at drawing blocks of stone.
Source Lazur URH
Uses spirals from Pixabay. To get the basic tile select the rectangle in Inkscape and use shift+alt+i
Source Firkin
A seamless paper background colored in pale yellow.
Source V. Hartikainen
As simple and subtle as it gets. But sometimes that’s just what you want.
Source Designova
A fun-looking elastoplast/band-aid pattern. A hint of orange tone in this one.
Source Josh Green
White fabric looking texture with some nice random wave features.
Source Hendrik Lammers
Zero CC plastic pattern texture, photographed and made by me. CC0 *Note, this texture was on the perfectly smooth surface of a plastic shovel scraper, not sure how to call it. Plz coment if you know what its called.
Source Sojan Janso
ZeroCC tileable stone texture, edited from pixabay. CC0
Source Sojan Janso
A seamless pattern formed from a square tile. To get the tile, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Hexagonal dark 3D pattern. What more can you ask for?
Source Norbert Levajsics
Not a pattern for fabrics, but one produced from a jpg of a stack of fabric items that was posted on Pixabay. The tile that this is based on can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
This one is super crisp at 2X. Lined paper with some dust and scratches.
Source HQvectors
A dark pattern made out of 3×3 circles and a 1px shadow. This works well as a carbon texture or background.
Source Atle Mo
People seem to enjoy dark patterns, so here is one with some circles.
Source Atle Mo
Zero CC Mossy stone tileable texture, photographed and made by me. CC0
Source Sojan Janso
A seamless pattern the unit cell for which can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin