A grid of squares with green colours. Since the colours are randomly distributed it is automatically seamless.
Source Firkin
From a drawing in 'Navigations de Alouys de Cademoste.-La Navigation du Capitaine Pierre Sintre', Alvise da ca da Mosto, 1895.
Source Firkin
It’s okay to be square! A nice light gray pattern with random squares.
Source Waseem Dahman
Dare I call this a «flat pattern»? Probably not.
Source Dax Kieran
You could get a bit dizzy from this one, but it might come in handy.
Source Dertig Media
Design drawn in Paint.net, vectorised using Vector Magic and finished in Inkscape.
Source Firkin
The classic 45-degree diagonal line pattern, done right.
Source Jorick van Hees
Everyone loves a diamond, right? Make your site sparkle.
Source AJ Troxell
The image depicts a seamless pattern of a fishnet with a plenty of fish.It may be a lucky charm for fishermen.
Source Yamachem
From a drawing in 'Gately's World's Progress', Charles Beale, 1886.
Source Firkin
Seamless pattern the tile for which can be had by using shift-alt-I on the selected rectangle in Inkscape.
Source Firkin
A repeatable image with dark background and metal grid pattern.
Source V. Hartikainen
Submitted in a cream color, but you know how I like it.
Source Devin Holmes
It looks like a polished stone surface to me. Download it for free, as always.
Source V. Hartikainen
Prismatic Basic Pattern 2 No Background
Source GDJ
From a drawing in 'Art Embroidery', M.S. Lockwood and E. Glaister, 1878.
Source Firkin