Might not be super subtle, but quite original in its form.
Source Alex Smith
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
You could get a bit dizzy from this one, but it might come in handy.
Source Dertig Media
This is the remix of an OCAL clipart called "Rain on Window" uploaded by "pagarmidna".Thanks.This is a seamless pattern of raindrops.
Source Yamachem
Formed by distorting the inside front cover of 'Diversæ insectarum volatilium : icones ad vivum accuratissmè depictæ per celeberrimum pictorem', Jacob Hoefnagel, 1630.
Source Firkin
A grid of squares with green colours. Since the colours are randomly distributed it is automatically seamless.
Source Firkin
A seamless paper background colored in pale yellow.
Source V. Hartikainen
Looks like a technical drawing board: small squares forming a nice grid.
Source We Are Pixel8
Drawn in Paint.net using the kaleidoscope plug-in and vectorised.
Source Firkin
Everyone needs some stardust. Sprinkle it on your next project.
Source Atle Mo
I know there is one here already, but this is sexy!
Source Gjermund Gustavsen
From a drawing in 'Worsborough; its historical associations and rural attractions', Joseph Wilkinson, 1879.
Source Firkin
The name Paisley reminds me of an old British servant. That’s just me.
Source Swetha
High detail stone wall with minor cracks and specks.
Source Projecteightyfive
An interesting dark spotted pattern at an angle.
Source Hendrik Lammers
Background formed from the original with an emboss effect
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
A dark one with geometric shapes and dotted lines.
Source Mohawk Studios
From a drawing in 'Sun Pictures of the Norfolk Broads', Ernest Suffling, 1892.
Source Firkin
Prismatic Floral Pattern 3 Variation 3 No Background
Source GDJ
A huge one at 800x600px. Made from a photo I took going home after work.
Source Atle Mo