Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
A dark metallic background with a pattern of stamped dots. Here's a dark "metallic" background pattern for you.
Source V. Hartikainen
Prismatic Curved Diamond Pattern 2 No Background
Source GDJ
A seamless background drawn in Paint.net and vectorised with Vector Magic. The starting point was a photograph of drinking straws from Pixabay.
Source Firkin
Zero CC tillable hard cover red book with X shape marks. Scanned and made by me.
Source Sojan Janso
You were craving more leather, so I whipped this up by scanning a leather jacket.
Source Atle Mo
Remixed from a drawing in 'An Index to Deering's Nottinghamia Vetus et Nova', Rupert Chicken, 1899. The unit tile can be had by selecting the rectangle in Inkscape and using shift-alt-i
Source Firkin
The name is totally random, but hey, it sounds good.
Source Atle Mo
Zero CC tileable bark texture, photographed and made by me. CC0
Source Sojan Janso
Sometimes you just need the simplest thing.
Source Fabricio
A seamless pattern formed from a square tile. The tile can be retrieved by selecting the rectangle in Inkscape and using shift-alt-I.
Source Firkin
A colourful background drawn originally in paint.net and vectorised in Vector Magic.
Source Firkin
ZeroCC tileable stone texture, edited from pixabay. CC0
Source Sojan Janso
From a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i
Source Firkin
Floral patterns will never go out of style, so enjoy this one.
Source Lasma
Based on several public domain drawings on Wikimedia Commons. This was formed from a rectangular tile. The tile can be accessed in Inkscape by selecting the rectangle and using shift-alt-i
Source Firkin
Black And White Floral Pattern Background Inverse
Source GDJ