A huge one at 800x600px. Made from a photo I took going home after work.
Source Atle Mo
A very dark asfalt pattern based off of a photo taken with my iPhone.
Source Atle Mo
A smooth mid-tone gray, or low contrast if you will, linen pattern.
Source Jordan Pittman
Zero CC tileable ground (#2) cracked, crackled texture, made by me. CC0
Source Sojan Janso
From a drawing in 'Friend or Fortune? The story of a strange year', Robert Overton, 1897.
Source Firkin
Prismatic Curved Diamond Pattern 7 No Background
Source GDJ
To get the tile this is based on, select the rectangle in Inkscape and use shift-alt-i.
Source Firkin
From a drawing in 'Studies for Stories', Jean Ingelow, 1864.
Source Firkin
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
A free seamless background pattern for use on websites.
Source V. Hartikainen
You don’t see many mid-tone patterns here, but this one is nice.
Source Joel Klein
This one is super crisp at 2X. Lined paper with some dust and scratches.
Source HQvectors
Just to prove my point, here is a slightly modified dark version.
Source Atle Mo
This one needs to be used in small areas; you can see it repeat.
Source Luca
There are many carbon patterns, but this one is tiny.
Source Designova
The image depicts an edo-era pattern called "same-komon" or "鮫小紋"which looks like a shark skin.The "same" in Japanese means shark in English.
Source Yamachem
Hexagonal dark 3D pattern. What more can you ask for?
Source Norbert Levajsics
Prismatic Isometric Cube Wireframe Pattern No Background
Source GDJ
From a drawing in 'Cowdray: the history of a great English House', Julia Roundell, 1884.
Source Firkin
It was called Navy Blue, but I made it dark. You know, the way I like it.
Source Ethan Hamilton
Heavily remixed from a drawing that was uploaded to Pixabay by ractapopulous
Source Firkin
After 1 comes 2, same but different. You get the idea.
Source Hendrik Lammers
Medium gray pattern with small strokes to give a weave effect.
Source Catherine
The name tells you it has curves. Oh yes, it does!
Source Peter Chon
Prismatic Geometric Pattern Background No Black
Source GDJ
Bright Multicolored Floral Background by Karen Arnold from PDP.
Source GDJ