Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
Medium gray fabric pattern with 45-degree lines going across.
Source Atle Mo
From an image on opengameart.org shared by rubberduck.
Source Firkin
Fabric-ish patterns are close to my heart. French Stucco to the rescue.
Source Christopher Buecheler
A seamless pattern formed from a square tile. The tile can be retrieved by selecting the rectangle in Inkscape and using shift-alt-I.
Source Firkin
A seamless pattern the unit cell for which can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Prismatic Polka Dots 3 No Background
Source GDJ
Number 4 in a series of 5 beautiful patterns. Can be found in colors on the submitter’s website.
Source Janos Koos
A dark gray, sandy pattern with small light dots, and some angled strokes.
Source Atle Mo
Your eyes can trip a bit from looking at this – use it wisely.
Source Michal Chovanec
Snap! It’s a pattern, and it’s not grayscale! Of course you can always change the color in Photoshop.
Source Atle Mo
A seamless pattern the unit cell for which can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin
Retro Circles Background 4 No Black
Source GDJ
From a drawing in 'Jardyne's Wife', Charles Wills, 1891.
Source Firkin
Prismatic Abstract Geometric Background 4
Source GDJ
Luxurious looking pattern (for a T-shirt maybe?) with a hint of green.
Source Simon Meek
Just like your old suit, all striped and smooth.
Source Alex Berkowitz
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
Bumps, highlight and shadows – all good things.
Source Badhon Ebrahim
Prismatic Curved Diamond Pattern 4 No Background
Source GDJ
Prismatic Abstract Line Art Pattern Background
Source GDJ
Zerro CC tillable texture of stones photographed and made by me. 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
From a drawing in 'Cowdray: the history of a great English House', Julia Roundell, 1884.
Source Firkin
This one is something special. I’d call it a flat pattern, too. Very well done, sir!
Source GetDiscount