The square tile this is based on can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin
A grid of squares with green colours. Since the colours are randomly distributed it is automatically seamless.
Source Firkin
From a drawing in 'Artists and Arabs', Henry Blackburn, 1868
Source Firkin
Stefan is hard at work, this time with a funky pattern of squares.
Source Stefan Aleksić
A seamless pattern drawn originally in Paint.net by distorting a slice of background pattern 116 and copying the resulting triangle numerous times.
Source Firkin
Prismatic Basic Pattern 2 No Background
Source GDJ
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
Seamless tile drawn in Paint.net and vectorised in Vector Magic.
Source Firkin
I know there is one here already, but this is sexy!
Source Gjermund Gustavsen
Prismatic Abstract Geometric Background 5
Source GDJ