Not so subtle. These tileable wood patterns are very useful.
Source Elemis
Prismatic 3D Isometric Tessellation Pattern 6
Source GDJ
This was formed by distorting an image of a background on Pixabay.
Source Firkin
A brown seamless wood texture in a form of stripe pattern. The result has turned out pretty well, in my opinion.
Source V. Hartikainen
Black brick wall pattern. Brick your site up!
Source Alex Parker
Geometric triangles seem to be quite hot these days.
Source Pixeden
From a drawing in 'Maidenhood; or, the Verge of the Stream', Laura Jewry, 1876.
Source Firkin
An emulated “transparent” background pattern, like that of all kinds of computer graphics software.
Source AdamStanislav
Seamless Prismatic Geometric Pattern With Background
Source GDJ
From a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Tiny little fibers making a soft and sweet look.
Source Badhon Ebrahim
It almost looks a bit blurry, but then again, so are fishes.
Source Petr Šulc
Might not be super subtle, but quite original in its form.
Source Alex Smith
Prismatic Chevrons Pattern 5 With Background
Source GDJ
Seamless , tileable CC-0 texture. Created by my own, feel free to use wherever you want!
Source Linolafett
ZeroCC tileable stone texture, edited from pixabay. CC0
Source Sojan Janso
A pattern drawn in Paint.net and vectorized in Vector Magic.
Source Firkin
Dark, square, clean and tidy. What more can you ask for?
Source Jaromír Kavan
It’s a hole, in a pattern. On your website. Dig it!
Source Josh Green
Square design drawn in Paint.net and vectorized in Vector Magic.
Source Firkin
Like the name says, light and gray, with some small dots and circles.
Source Brenda Lay
No relation to the band, but damn it’s subtle!
Source Thomas Myrman
From a drawing in 'Gately's World's Progress', Charles Beale, 1886.
Source Firkin
Actually remixed from a pattern on Pixabay. But then noticed a very similar one on Openclipart.org uploaded by btj51q2.
Source Firkin