Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
The tile this fill pattern is based on can be had by using shift+alt+i on the rectangle.
Source Firkin
A blue background wallpaper for websites. It has a seamless texture with vertical stripes. It looks quite nice not only when using as a tiled background on websites, but also on computer desktops.
Source V. Hartikainen
Semi-light fabric pattern made out of random pixels in shades of gray.
Source Atle Mo
From a drawing in 'Line and form', Walter Crane, 1914.
Source Firkin
Colour version that is close to the original drawing uploaded to Pixabay by pencilparker.
Source Firkin
As far as fabric patterns goes, this is quite crisp.
Source Heliodor Jalba
From a drawing in 'Art Embroidery', M.S. Lockwood and E. Glaister, 1878.
Source Firkin
Zero CC tileable grass texture, photographed and made by me. CC0
Source Sojan Janso
Zero CC tileable yellow craft paper; scanned and made by me. CC0
Source Sojan Janso
Adapted heavily from a JPG that was uploaded to Pixabay by Viscious-Speed.
Source Firkin
From a drawing in 'Resa i Afrika, genom Angola, Ovampo och Damaraland', P. Moller, 1899.
Source Firkin
The file was named striped lens, but hey – Translucent Fibres works too.
Source Angelica
Formed from decorative divider 184 in paint.net. Vectorised with Vector Magic.
Source Firkin
A large pattern with funky shapes and form. An original. Sort of origami-ish.
Source Luuk van Baars
Drawn in Paint.net using the kaleidoscope plug-in and vectorised.
Source Firkin
Sharp pixel pattern, just like the good old days.
Source Paridhi
Background formed from the iconic plastic construction bricks that gave me endless hours of fun when I was a lad.
Source Firkin
A car pattern?! Can it be subtle? I say yes!
Source Radosław Rzepecki
Did anyone say The Hoff? This pattern is in no way related to Baywatch.
Source Josh Green
This is so subtle you need to bring your magnifier!
Source Carlos Valdez
A pattern drawn in Paint.net and vectorized in Inkscape.
Source Firkin
A free seamless background image with abstract texture of green "curtain".
Source V. Hartikainen