A seamless light gray paper texture with horizontal double lines.
Source V. Hartikainen
Sort of like the Photoshop transparent background, but better!
Source Alex Parker
Dark blue concrete wall with some small dust spots.
Source Atle Mo
This one is rather fun and playful. The 2X could be used at 1X too!
Source Welsley
Remixed from a drawing in 'Paul's Sister', Frances Peard, 1889.
Source Firkin
Seamless Background For Websites. It has a texture similar to cork-board.
Source V. Hartikainen
Formed by heavily distorting part of a an image of a fish uploaded to Pixabay by GLady
Source Firkin
Derived from a JPG that was uploaded to Pixabay by ractapopulous
Source Firkin
From a drawing in 'Art Embroidery', M.S. Lockwood and E. Glaister, 1878.
Source Firkin
A seamless pattern recreated from an image on Pixabay. It is reminiscent of parquet flooring and is formed from a square tile, which can be recovered in Inkscape by selecting the ungrouped rectangle and using shift-alt-I together.
Source Firkin
Super detailed 16×16 tile that forms a beautiful pattern of straws.
Source Pavel
Horizontal and vertical lines on a light gray background.
Source Adam Anlauf
Derived from a corner decoration itself found as a jpg on Pixabay.
Source Firkin
The unit cell for this seamless pattern can be had in Inkscape by selecting the rectangle and using shift+alt+i.
Source Firkin
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
Remixed from a drawing in 'Analecta Eboracensia', Thomas Widdrington, 1897.
Source Firkin
From a drawing in 'Heroes of North African Discovery', Nancy Meugens, 1894.
Source Firkin
Prismatic Polyskelion Pattern No Background
Source GDJ
Continuing the geometric trend, here is one more.
Source Mike Warner