The tile this fill pattern is based on can be had by using shift+alt+i on the rectangle.
Source Firkin
From a drawing in 'At home', J. Sowerby, J. Crane and T. Frederick, 1881.
Source Firkin
This background pattern contains a seamless texture of bark. It's not very realistic, but I think it looks quite nice.
Source V. Hartikainen
A seamless marble-like texture colored in light blue.
Source V. Hartikainen
The image depicts the Japanese Edo pattern called "seigaiha" or "青海波" meaning "blue -sea- wave".I hope it's suitable for the summer season.
Source Yamachem
This is so subtle I hope you can see it! Tweak at will.
Source Alexandre Naud
CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
Remixed from a drawing in 'Hungary. A guide book. By several authors', 1890.
Source Firkin
Alternative colour scheme. 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
Super detailed 16×16 tile that forms a beautiful pattern of straws.
Source Pavel
Semi-light fabric pattern made out of random pixels in shades of gray.
Source Atle Mo
The image depicts a seamless pattern of a tortoise in tortoiseshell (hexagon).
Source Yamachem
Remixed from a drawing in 'Line and form', Walter Crane, 1914.
Source Firkin
Prismatic Polka Dots Mark II 3 No Background
Source GDJ
A seamless design of flowers remixed from a jpg on Pixabay by Prawny.
Source Firkin
Prismatic Floral Pattern 3 Variation 3 No Background
Source GDJ
A seamless dark leather-like background texture with diagonal lines that look like stitches.
Source V. Hartikainen
You know, tiny and sharp. I’m sure you’ll find a use for it.
Source Atle Mo
Bright Multicolored Floral Background by Karen Arnold from PDP.
Source GDJ
Remixed from a design seen in 'Burghley. The Life of William Cecil', William Charlton, 1857.
Source Firkin
Embossed lines and squares with subtle highlights.
Source Alex Parker
This was formed by distorting an image of a background on Pixabay.
Source Firkin
Remixed from a drawing in 'Hungary. A guide book. By several authors', 1890.
Source Firkin
Inspired by a pattern found in 'A General History of Hampshire, or the County of Southampton, including the Isle of Wight', Bernard Woodwood, 1861
Source Firkin
An emulated “transparent” background pattern, like that of all kinds of computer graphics software.
Source AdamStanislav