Black Mamba #57
 Dark  CC BY-SA 3.0

The name alone is awesome, but so is this sweet dark pattern.

Source Federica Pelzel

 More Textures
Tessellation 9 #2548
 Grid  CC 0

A seamless tessellation pattern. To get the tile this is formed from, select the pattern in Inkscape and use shift+alt+i.

Source Firkin

Configurable Graph Paper #141
 Stripes  CC BY-SA 3.0

Pass parameters to the URL or edit the source code variables to configure the graph paper for the division desired.

Source JayNick

Gray Sand@2X #18
 Dark  CC BY-SA 3.0

A dark gray, sandy pattern with small light dots, and some angled strokes.

Source Atle Mo

Background pattern 261 #2076
 Dark  CC 0

Inspired by a pattern found in 'A General History of Hampshire, or the County of Southampton, including the Isle of Wight', Bernard Woodwood, 1861

Source Firkin

Background pattern 308 (colour 5) #1869
 Blue  CC 0

Design drawn in Paint.net, vectorised using Vector Magic and finished in Inkscape.

Source Firkin

Prismatic Groovy Concentric Background 2@2X #454
 Noise  CC 0

Prismatic Groovy Concentric Background 2

Source GDJ

Green Fibers@2X #92
 Dark  CC BY-SA 3.0

The green fibers pattern will work very well in grayscale as well.

Source Matteo Di Capua

Dark Denim #29
 Dark  CC BY-SA 3.0

A dark denim looking pattern. 145×145 pixels.

Source Atle Mo