Snap! It’s a pattern, and it’s not grayscale! Of course you can always change the color in Photoshop.
Source Atle Mo
Remixed from a drawing in 'Очерки Русской Исторіи въ памятникахъ быта', Petr Polevoi, 1879.
Source Firkin
Prismatic Groovy Concentric Background
Source GDJ
The unit cell for this seamless pattern can be had in Inkscape by selecting the rectangle and using shift+alt+i.
Source Firkin
Dare I call this a «flat pattern»? Probably not.
Source Dax Kieran
Seamless , tileable CC-0 texture. Created by my own, feel free to use wherever you want!
Source Linolafett
Background formed from the original with an emboss effect.
Source Firkin
Zero CC asphalt, pavement, texture, photographed and made by me. CC0 WARNING I FOUND A SEAM ON THIS TEXTURE
Source Sojan Janso
Prismatic Groovy Concentric Background 5 No Black
Source GDJ
ZeroCC tileable moss texture, photographed and made by me. CC0
Source Sojan Janso
A background pattern with green vertical stripes. A new striped background pattern. This time a green one.
Source V. Hartikainen
A set of paper filters. The base texture is generated the same way, only the compositing mode is varied.
Source Lazur URH
A comeback for you: the popular Escheresque, now in black.
Source Patten
Retro Circles Background 4 No Black
Source GDJ
A seamlessly tile-able grunge background image.
Source V. Hartikainen
The unit cell for this seamless pattern can be had in Inkscape by selecting the rectangle and using shift+alt+i.
Source Firkin
Light square grid pattern, great for a “DIY projects” sort of website, maybe?
Source Rafael Almeida
A seamless green background texture. The image is distributed under a Creative Commons License (like all of the images here).
Source V. Hartikainen
Not the most creative name, but it’s a good all-purpose light background.
Source Dmitry
From a drawing in 'Sun Pictures of the Norfolk Broads', Ernest Suffling, 1892.
Source Firkin