Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
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
More carbon fiber for your collections. This time in white or semi-dark gray.
Source Badhon Ebrahim
A seamless pattern based on a rectangular tile that can be retrieved in Inkscape by selecting the rectangle and using shift-alt-i.
Source Firkin
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
Simple gray checkered lines, in light tones.
Source Radosław Rzepecki
Derived from an image that was uploaded to Pixabay by TheDigitalArtist
Source Firkin
Background Wall, Art Abstract, Star Well & CC0 texture.
Source Ractapopulous
A seamless texture of worn out "cardboard".
Source V. Hartikainen
A beautiful dark wood pattern, superbly tiled.
Source Omar Alvarado
Heavy depth and shadows here, but might work well on some mobile apps.
Source Damian Rivas
Seamless Prismatic Quadrilateral Line Art Pattern No Background
Source GDJ
To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i
Source Firkin
Prismatic Groovy Concentric Background 2
Source GDJ
Black paper texture, based on two different images.
Source Atle Mo
Based from Design Kindle
Light gray paper pattern with small traces of fiber and some dust.
Source Atle Mo
Square design drawn in Paint.net and vectorized in Vector Magic.
Source Firkin
Prismatic Abstract Geometric Background 4 No Black
Source GDJ
An orange vertically striped background pattern. Feel free to download and use this orange background pattern, for example, on the web). It resembles a wallpaper with vertical stripes or something similar to it.
Source V. Hartikainen
To get the tile this is formed from select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
A beautiful dark wood pattern, superbly tiled.
Source Omar Alvarado