Did some testing with Repper Pro tonight, and this gray mid-tone pattern came out.
Source Atle Mo
Thin lines, noise and texture creates this crisp dark denim pattern.
Source Marco Slooten
Derived from a PNG that was uploaded to Pixabay by nutkitten
Source Firkin
A grid of squares with green colours. Since the colours are randomly distributed it is automatically seamless.
Source Firkin
A background pattern inspired by designs seen in 'Burghley. The Life of William Cecil', William Charlton, 1857.
Source Firkin
Very dark pattern with some noise and 45-degree lines.
Source Stefan Aleksić
An abstract texture of water. It's not perfect, but will do. You may download if you like it.
Source V. Hartikainen
Prismatic Rounded Squares Grid 4 No Background
Source GDJ
One more in the line of patterns inspired by Japanese/Asian styles. Smooth.
Source Kim Ruddock
Tile available in Inkscape using shift-alt-i on the selected rectangle
Source Firkin
The square tile this is based on can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin
This is so subtle you need to bring your magnifier!
Source Carlos Valdez
To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Utilising some flowers from Almeidah. To get the unit tile, select the rectangle in Inkscape and use shift-alt-i.
Source Firkin
A seamless striped fabric-like texture colored in a dark reddish brown color.
Source V. Hartikainen
The first pattern on here using opacity. Try it on a site with a colored background, or even using mixed colors.
Source Nathan Spady
Got some felt in my mailbox today, so I scanned it for you to use.
Source Atle Mo
Light square grid pattern, great for a “DIY projects” sort of website, maybe?
Source Rafael Almeida
One of the few full-color patterns here, but this one was just too good to pass up.
Source Alexey Usoltsev
From a drawing in 'Storia del Palazzo Vecchio in Firenze', Aurelio Gotti, 1889.
Source Firkin
This is a remix of "flower seamless pattern".I rotated the original image by 90 degrees.This is a seamless pattern of flowers.These horizontal wavy lines are one of Edo patterns which is called "tatewaku or tachiwaku or 立湧" that represents uprising steam or vapor.
Source Yamachem