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
Actually remixed from a pattern on Pixabay. But then noticed a very similar one on Openclipart.org uploaded by btj51q2.
Source Firkin
Prismatic Abstract Background Design
Source GDJ
Prismatic Abstract Line Art Pattern Background 2
Source GDJ
From a drawing in 'At home', J. Sowerby, J. Crane and T. Frederick, 1881.
Source Firkin
A subtle shadowed checkered pattern. Increase the lightness for even more subtle sexiness.
Source Josh Green
A bit like some carbon, or knitted netting if you will.
Source Anna Litvinuk
Super subtle indeed, a medium gray pattern with tiny dots in a grid.
Source Designova
Zero CC tileable wood texture, made by me procedurally in Neo Texture Edit.
Source Sojan Janso
One more in the line of patterns inspired by Japanese/Asian styles. Smooth.
Source Kim Ruddock
Remixed from a drawing in 'The March of Loyalty', Letitia MacClintock, 1884.
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 'La Principauté de Liège et les Pays-Bas au XVIe siècle', Société des Bibliophiles Liégeois ,1887.
Source Firkin
The first pattern on here using opacity. Try it on a site with a colored background, or even using mixed colors.
Source Nathan Spady
Prismatic Curved Diamond Pattern 7 No Background
Source GDJ
From a drawing in 'Real Sailor-Songs', John Ashton, 1891.
Source Firkin
Turn your site into a dragon with this great scale pattern.
Source Alex Parker
Pixel by pixel, sharp and clean. Very light pattern with clear lines.
Source M.Ashok
Sometimes simple really is what you need, and this could fit you well.
Source Factorio.us Collective
Oh yes, it happened! A pattern in full color.
Source Atle Mo
Made by distorting a simple pattern using the 'sin waves' plugin for Paint.net and vectorising in Vector Magic
Source Firkin