Soft Kill #318
 Dark  CC BY-SA 3.0

Pattern #100! A black classic knit-looking pattern.

Source Factorio.us Collective

 More Textures
Background pattern black #2028
 Dark  CC 0

Remixed from a design seen in 'Burghley. The Life of William Cecil', William Charlton, 1857. The tile this is based on can be had by selecting the rectangle in Inkscape and using shift+alt+i.

Source Firkin

Gun Metal #360
 Dark  CC BY-SA 3.0

With a name this awesome, how can I go wrong?

Source Nikolay Boltachev

Double Lined@2X #52
 Stripes  CC BY-SA 3.0

Horizontal and vertical lines on a light gray background.

Source Adam Anlauf

Abstract Tiled Background Extended 12 #523
 Noise  CC 0

Abstract Tiled Background Extended 12

Source GDJ

Prismatic Groovy Concentric Background 5 #465
 Dark  CC 0

Prismatic Groovy Concentric Background 5

Source GDJ

Greek key pattern 5 #2294
 Dark  CC 0

Seamless pattern made from a tile that can be obtained in Inkscape by selecting the rectangle and using shift+alt+i.

Source Firkin

White carbon@2X #12
 Carbon  CC BY-SA 3.0

Same as the black version, but now in shades of gray. Very subtle and fine grained.

Source Atle Mo

Candyhole #356
 Light  CC BY-SA 3.0

It’s a hole, in a pattern. On your website. Dig it!

Source Josh Green

Prismatic Abstract Geometric Background 3 #270
 Noise  CC BY-SA 3.0

Prismatic Abstract Geometric Background 3

Source GDJ