CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
A heavy dark gray base, some subtle noise and a 45-degree grid makes this look like a pattern with a tactile feel to it.
Source Atle Mo
A dark metal plate with an embossed grid pattern and a bit of rust. Here's a dark metal plate texture for use as a tiled background on web pages.
Source V. Hartikainen
Fix and cc0 to get the tile this is based on.
Source SliverKnight
From a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
From a drawing in 'Navigations de Alouys de Cademoste.-La Navigation du Capitaine Pierre Sintre', Alvise da ca da Mosto, 1895.
Source Firkin
From a drawing in 'Maidenhood; or, the Verge of the Stream', Laura Jewry, 1876.
Source Firkin
Remixed from a drawing in 'A Girl in Ten Thousand', Elizabeth Meade, 1896.
Source Firkin
Prismatic Abstract Geometric Background 4 No Black
Source GDJ
Prismatic Snowflakes Pattern 3 No Background
Source GDJ
This is so subtle I hope you can see it! Tweak at will.
Source Alexandre Naud
Based on an image that was uploaded to Pixabay by devanath
Source Firkin
This was formed by distorting an image of a background on Pixabay.
Source Firkin
A very slick dark rubber grip pattern, sort of like the grip on a camera.
Source Sinisha
A nice and simple gray stucco material. Great on its own, or as a base for a new pattern.
Source Bartosz Kaszubowski
This one is rather fun and playful. The 2X could be used at 1X too!
Source Welsley
This was formed by distorting an image of a background on Pixabay.
Source Firkin
A bit simplified version. Although it could be edited out to be simpler. Anyway, this time the tiling is converted to a pattern fill -which is using clipping for the tile's edges.
Source Lazur URH
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
Number five from the same submitter, makes my job easy.
Source Dima Shiper
Seamless pattern formed from a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin