From a drawing in 'Line and form', Walter Crane, 1914.
Source Firkin
A seamless texture of a rough concrete surface.
Source V. Hartikainen
Seamless Prismatic Quadrilateral Line Art Pattern No Background
Source GDJ
The tile this is formed from can be retrieved in Inkscape by selecting the rectangle and using shift+alt+i
Source Firkin
From a drawing in 'In an Enchanted Island', William Mallock, 1892.
Source Firkin
Paper pattern with small dust particles and 45-degree strokes.
Source Atle Mo
The tile this is based on can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin
CC0 and seamless wellington boot pattern.
Source SliverKnight
Dark Tile-able Grunge Texture. I think this texture can be classified as grunge. It's free and seamless, as always.
Source V. Hartikainen
Prismatic Groovy Concentric Background No Black
Source GDJ
Retro Circles Background 7 No Black
Source GDJ
From an image on opengameart.org shared by rubberduck.
Source Firkin
U.S.-based National Fire Protection Association standard fire diamond for flagging risks posed by hazardous materials. The red diamond has a number 0-4 depending on flammability. The blue diamond has a number 0-4 depending on health hazard. The yellow has a number 0-4 depending on reactivity. the white square has a special notice, e.g OX for oxidizer.
Source Firkin
Here's a seamless brown cork board background texture. Feel free to download or reshare if you like.
Source V. Hartikainen
Number five from the same submitter, makes my job easy.
Source Dima Shiper
A dark pattern made out of 3×3 circles and a 1px shadow. This works well as a carbon texture or background.
Source Atle Mo
From a design found in 'History of the Virginia Company of London; with letters to and from the first Colony, never before printed', Edward Neill, 1869.
Source Firkin
It has waves, so make sure you don’t get sea sickness.
Source CoolPatterns
To celebrate the new feature, we need some sparkling diamonds.
Source Atle Mo