The unit cell for this seamless pattern can be had in Inkscape by selecting the rectangle and using shift+alt+i.
Source Firkin
A version without colours blended together to give a different look.
Source Firkin
Prismatic Groovy Concentric Background 2 No Black
Source GDJ
Light gray paper pattern with small traces of fiber and some dust.
Source Atle Mo
He influenced us all. “Don’t be sad because it’s over. Smile because it happened.”
Source Atle Mo
This is so subtle you need to bring your magnifier!
Source Carlos Valdez
This one looks like a cork panel. Feel free to use it as a tiled background on your blog or website.
Source V. Hartikainen
A dark background pattern/texture of a dimpled metal plate.
Source V. Hartikainen
A grid of squares with green colours. Since the colours are randomly distributed it is automatically seamless.
Source Firkin
Zero CC tileable pine bark texture, photographed and made by me. CC0
Source Sojan Janso
From a drawing in 'From Snowdon to the Sea. Striking stories of North and South Wales', Marie Trevelyan, 1895.
Source Firkin
A beautiful dark wood pattern, superbly tiled.
Source Omar Alvarado
Love the style on this one, very fresh. Diagonal diamond pattern. Get it?
Source INS
Can never have too many knitting patterns, especially as nice as this.
Source Victoria Spahn
Seamless , tileable CC-0 texture. Created by my own, feel free to use wherever you want!
Source Linolafett
Prismatic Groovy Concentric Background 2
Source GDJ
Prismatic Floral Background No Black
Source GDJ
Small dots with minor circles spread across to form a nice mosaic.
Source John Burks
From a drawing in 'Artists and Arabs', Henry Blackburn, 1868
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