Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
Black paper texture, based on two different images.
Source Atle Mo
Based from Design Kindle
Prismatic Floral Pattern 3 Variation 3 No Background
Source GDJ
A seamless pattern based on a square tile that can be retrieved in Inkscape by selecting the rectangle and using shift-alt-i.
Source Firkin
Super simple but very nice indeed. Gray with vertical stripes.
Source Merrin Macleod
The image is the remix of "wire-mesh fence seamless pattern" .This is a more minute version of it.Sorry for the file size.Using path>difference in Inkscape, I will cut out any silhouette from this pattern and create a "meshed silhouette".
Source Yamachem
This is the remix of an OCAL clipart called "Art Nouveau ornament" uploaded by "microcosme".Thanks.This is a seamless pattern of an Art Nouveau ornament.
Source Yamachem
Zero CC tileable dry grass texture, photographed and made by me. CC0
Source Sojan Janso
An interesting dark spotted pattern at an angle.
Source Hendrik Lammers
Small dots with minor circles spread across to form a nice mosaic.
Source John Burks
Super detailed 16×16 tile that forms a beautiful pattern of straws.
Source Pavel
The basic shapes never get old. Simple triangle pattern.
Source Atle Mo
Based on an image that was uploaded to Pixabay by devanath
Source Firkin
Not so subtle. These tileable wood patterns are very useful.
Source Elemis
A seamlessly tile-able grunge background image.
Source V. Hartikainen
Seamless pattern formed from a tile that can be had by selecting the rectangle in Inkscape and using shift-alt-i.
Source Firkin
It almost looks a bit blurry, but then again, so are fishes.
Source Petr Šulc
Not even 1kb, but very stylish. Gray thin lines.
Source Struck Axiom
Design drawn in Paint.net, vectorised using Vector Magic and finished in Inkscape.
Source Firkin
A free black metallic background pattern. Here's a new pattern I made that looks metallic.
Source V. Hartikainen
Prismatic Groovy Concentric Background 3
Source GDJ
A pattern formed from a squared tile. The tile can be accessed in Inkscape by selecting the rectangle and using shift-alt-i.
Source Firkin