A comeback for you: the popular Escheresque, now in black.
Source Patten
Snap! It’s a pattern, and it’s not grayscale! Of course you can always change the color in Photoshop.
Source Atle Mo
From a drawing in 'At home', J. Sowerby, J. Crane and T. Frederick, 1881.
Source Firkin
A car pattern?! Can it be subtle? I say yes!
Source Radosław Rzepecki
Seamless Prismatic Quadrilateral Line Art Pattern No Background
Source GDJ
Inspired by a 1930s wallpaper pattern I saw on TV.
Source Firkin
White little knobs, coming in at 10x10px. Sweet!
Source Amos
By popular request, an outline version of the pentagon pattern.
Source Atle Mo
Not even 1kb, but very stylish. Gray thin lines.
Source Struck Axiom
A free seamless background pattern for use on websites.
Source V. Hartikainen
Simple wide squares with a small indent. Fits all.
Source Petr Šulc.
Zero CC tileable hard cover red book, scanned and made by me. CC0
Source Sojan Janso
Remixed from an image that was uploaded to Pixabay by Pixeline
Source Firkin
A seamless pattern with green and yellow diagonal lines on top of a white dotted background.
Source V. Hartikainen
Background pattern originally a PNG drawn in Paint.net
Source Firkin
This background image has seamless texture that resembles a surface of gray stone.
Source V. Hartikainen
Neat little photography icon pattern.
Source Hossam Elbialy
The classic 45-degree diagonal line pattern, done right.
Source Jorick van Hees
A seamlessly repeating background pattern of wood. The image is procedurally generated, and, I think, it's turned out quite well.
Source V. Hartikainen
Prismatic Curved Diamond Pattern 2 No Background
Source GDJ
Number 1 in a series of 5 beautiful patterns. Can be found in colors on the submitter’s website.
Source Janos Koos
From a drawing in 'Sun Pictures of the Norfolk Broads', Ernest Suffling, 1892.
Source Firkin
Recreated from a pattern found in 'Az Osztrák-Magyar Monarchia irásban és képben', 1882. To get the tile this is based on select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Zero CC tileable wood texture, made by me procedurally in Neo Texture Edit.
Source Sojan Janso
Vector version of a png that was uploaded to Pixabay by pencilparker
Source Firkin
A monochrome pattern from a tile that can be had by selecting the rectangle in Inkscaope and using shift+alt+i
Source Firkin
Inspired by a 1930s wallpaper pattern I saw on TV.
Source Firkin
Not a flat you live inside, like in the UK – but a flat piece of cardboard.
Source Appleshadow
From a drawing in 'Line and form', Walter Crane, 1914.
Source Firkin