Background Wall, Art Abstract, Block Well & CC0 texture.
Source Ractapopulous
From a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Prismatic Curved Diamond Pattern 4 No Background
Source GDJ
Remixed from an image that was uploaded to Pixabay by KirstenStar
Source Firkin
Dark Tile-able Grunge Texture. I think this texture can be classified as grunge. It's free and seamless, as always.
Source V. Hartikainen
Sharp pixel pattern, just like the good old days.
Source Paridhi
Sort of like the Photoshop transparent background, but better!
Source Alex Parker
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
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
Seamless pattern formed from a tile that can be extracted by selecting the rectangle in Inkscape and using shift-alt-i.
Source Firkin
This is a more minute version of "fishnet 01".The image depicts a seamless pattern of a fishnet with a plenty of fish.It may be a lucky charm for fishermen.
Source Yamachem
A simple example on using clones. You can generate a nice base for a pattern fill quickly with it.
Source Lazur URH
From a drawing in 'Line and form', Walter Crane, 1914.
Source Firkin
This is a hot one. Small, sharp and unique.
Source GraphicsWall
Looks like an old rug or a computer chip.
Source Patutin Sergey
Could remind you a bit of those squares in Super Mario Bros, yeh?
Source Jeff Wall
From a drawing in 'Les Chroniqueurs de l'Histoire de France depuis les origines jusqu'au XVIe siècle', Henriette Witt, 1884.
Source Firkin
Detailed but still subtle and quite original. Lovely gray shades.
Source Kim Ruddock
A good starting point for a cardboard pattern. This would work well in a variety of colors.
Source Atle Mo
A seamless green background texture. The image is distributed under a Creative Commons License (like all of the images here).
Source V. Hartikainen
Sharp diamond pattern. A small 24x18px tile.
Source Tom Neal
Background Wall, Art Abstract, Block Well & CC0 texture.
Source Ractapopulous
Recreated from a pattern found in 'Az Osztrák-Magyar Monarchia irásban és képben', 1882. To get the tile this is based on select the rectangle in Inkscape and use shift+alt+i.
Source Firkin