More Textures
Transparent Square Tiles #298
 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

Background pattern purple #1998
 Colorful  CC 0

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

Source Firkin

Mosaic tile #2481
 Brown  CC 0

From a drawing in 'Handbook of the excursions proposed to be made by the Lincoln Diocesan Architectural Society, on the 27th and 28th of May, 1857', Edward Trollope, 1857.

Source Firkin

Background pattern 6 #226
 Noise  CC 0

A pattern derived from part of a fractal rendering in Paint.net.

Source Firkin

“Transparent” Background Pattern #493
 Light  CC 0

An emulated “transparent” background pattern, like that of all kinds of computer graphics software.

Source AdamStanislav

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

My Little Plaid #332
 Light  CC BY-SA 3.0

Repeating squares overlapping.

Source Pete Fecteau

Retro Squares Background 4@2X #419
 Dark  CC 0

Retro Squares Background 4

Source GDJ

Floral Abstract Pattern Background #217
 Noise  CC 0

PDP

Source GDJ

Noisy@2X #46
 Paper  CC BY-SA 3.0

Looks a bit like concrete with subtle specks spread around the pattern.

Source Mladjan Antic