A seamless pattern the unit cell for which can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Drawn in Paint.net using the kaleidoscope plug-in and vectorised.
Source Firkin
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
A background formed from an image of an old tile on the New York Metropolitan Museum of Art website. To get the base tile, select the rectangle in Inkscape and use shift-alt-i.
Source Firkin
Remixed from an image that was uploaded to Pixabay by CatherineClennan
Source Firkin
Looks like an old wall. I guess that’s it then?
Source Viahorizon
Seamless SVG vector and JPG backgrounds with faded diagonal stripes. The colors are editable.
Source V. Hartikainen
Prismatic Groovy Concentric Background 5 No Black
Source GDJ
Submitted in a cream color, but you know how I like it.
Source Devin Holmes
Don’t look at this one too long if you’re high on something.
Source Luuk van Baars
Prismatic Geometric Tessellation Pattern 3 No Background
Source GDJ
A seamless pattern the starting point for which was a 'rainbow twist' texture in Paint.net.
Source Firkin
Oh yes, it happened! A pattern in full color.
Source Atle Mo
Sometimes you just need the simplest thing.
Source Fabricio
Nicely crafted paper pattern, although a bit on the large side (500x593px).
Source Blaq Annabiosis
From a drawing in 'Uit de geschiedenis der Heilige Stede te Amsterdam', Yohannes Sterck, 1898.
Source Firkin
Inspired by a pattern I saw in a 19th century book. This seamless pattern was created from a square tile. To get the tile, select the pattern in Inkscape and use shift+alt+i.
Source Firkin