Colorful Geometric Pattern Background #231
 Noise  CC 0

PDP

Source GDJ

 More Textures
Cardboard@2X #279
 Paper  CC BY-SA 3.0

A good starting point for a cardboard pattern. This would work well in a variety of colors.

Source Atle Mo

Grunge Wall #81
 Wall  CC BY-SA 3.0

Light gray grunge wall with a nice texture overlay.

Source Adam Anlauf

White carbon #11
 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

Floral Abstract Pattern Background #217
 Noise  CC 0

PDP

Source GDJ

Subtle Freckles@2X #90
 Light  CC BY-SA 3.0

This is so subtle: We’re talking 1% opacity. Get your squint on!

Source Atle Mo

Prismatic Chevrons Pattern 5 With Background@2X #470
 Dark  CC 0

Prismatic Chevrons Pattern 5 With Background

Source GDJ

Vintage pattern #2296
 Brown  CC 0

Inspired by a pattern seen on a public domain image of a very old tile. To get the unit cell, select the rectangle in Inkscape and use shift+alt+i.

Source Firkin

Background pattern 267 #2067
 Dark  CC 0

Remixed from a drawing in 'An Index to Deering's Nottinghamia Vetus et Nova', Rupert Chicken, 1899. The unit tile can be had by selecting the rectangle in Inkscape and using shift-alt-i

Source Firkin

Seamless Cardboard Texture #1212
 Cardboard  CC BY-SA 3.0

A free background image with a seamless texture of cardboard. This texture of cardboard looks quite realistic, especially when is actually tiled.

Source V. Hartikainen

Colourful background #2010
 Colorful  CC 0

Made by distorting a simple pattern using the 'sin waves' plugin for Paint.net and vectorising in Vector Magic

Source Firkin

Brushed Alum Dark@2X #66
 Dark  CC BY-SA 3.0

Because I love dark patterns, here is Brushed Alum in a dark coating.

Source Tim Ward

Concrete Wall 2 #79
 Wall  CC BY-SA 3.0

A light gray wall or floor (you decide) of concrete.

Source Atle Mo

Random Grey Variations@2X #70
 Stripes  CC BY-SA 3.0

Stefan is hard at work, this time with a funky pattern of squares.

Source Stefan Aleksić