Transparent Square Tiles@2X #299
 Grid  CC BY-SA 3.0

The first pattern on here using opacity. Try it on a site with a colored background, or even using mixed colors.

Source Nathan Spady

 More Textures
Always Grey@2X #54
 Dark  CC BY-SA 3.0

Crossing lines with a subtle emboss effect on a dark background.

Source Stefan Aleksić

Prismatic Dots Background #501
 Noise  CC 0

Prismatic Dots Background

Source GDJ

My Little Plaid@2X #333
 Light  CC BY-SA 3.0

Repeating squares overlapping.

Source Pete Fecteau

Prismatic Groovy Concentric Background 5 No Black@2X #468
 Dark  CC 0

Prismatic Groovy Concentric Background 5 No Black

Source GDJ

Whitey #109
 Light  CC BY-SA 3.0

A white version of the very popular linen pattern.

Source Ant Ekşiler

Geometric pattern #2003
 Grid  CC 0

A monochrome pattern from a tile that can be had by selecting the rectangle in Inkscaope and using shift+alt+i

Source Firkin

Background pattern 66 #195
 Wall  CC 0

A background pattern drawn in Inkscape.

Source Firkin

Decorative divider 283 (version 2) #1855
 Dark  CC 0

Remixed from a drawing in 'In an Enchanted Island', William Mallock, 1892.

Source Firkin

Background pattern 238 (colour 3) #2227
 Blue  CC 0

To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i.

Source Firkin

Vintage pattern 2 #2295
 Yellow  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