Might not be super subtle, but quite original in its form.
Source Alex Smith
The image depicts meshed silhouettes of various things.The original image is an OCAL clipart called "Enter FOSSASIA 2016 #IoT T-shirt Design Contest" uploaded by "openclipart".Thanks.
Source Yamachem
Your eyes can trip a bit from looking at this – use it wisely.
Source Michal Chovanec
Submitted by DomainsInfo – wtf, right? But hey, a free pattern.
Source DomainsInfo
Seamless , tileable CC-0 texture. Created by my own, feel free to use wherever you want!
Source Linolafett
Sounds French. Some 3D square diagonals, that’s all you need to know.
Source Graphiste
An abstract pale yellow paper-like background with stains colored in yellow and green.
Source V. Hartikainen
Here's a seamless brown cork board background texture. Feel free to download or reshare if you like.
Source V. Hartikainen
Seamless Prismatic Pythagorean Line Art Pattern No Background. A seamless pattern that includes the original tile (go to Objects / Pattern / Pattern To Objects in Inkscape's menu to extract it).
Source GDJ
A seamless pattern the starting point for which was a 'light rays' rendering in Paint.net.
Source Firkin
From a drawing in 'Resa i Afrika, genom Angola, Ovampo och Damaraland', P. Moller, 1899.
Source Firkin
From a drawing in 'Gately's World's Progress', Charles Beale, 1886.
Source Firkin
A seamless light gray paper texture with horizontal double lines.
Source V. Hartikainen
Not so subtle. These tileable wood patterns are very useful.
Source Elemis
A topographic map like this has actually been requested a few times, so here you go!
Source Sam Feyaerts
Pixel by pixel, sharp and clean. Very light pattern with clear lines.
Source M.Ashok