Seamless pattern the basic tile for which can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Like the name says, light and gray, with some small dots and circles.
Source Brenda Lay
Sort of like the Photoshop transparent background, but better!
Source Alex Parker
Formed from a tile based on a drawing from 'Viaggi d'un artista nell'America Meridionale', Guido Boggiani, 1895.
Source Firkin
Seamless Prismatic Pythagorean Line Art Pattern No Background. A seamless pattern that includes the original tile (go to Objects / Pattern / Pattern To Objects in Inkscape's menu to extract it).
Source GDJ
This background texture resembles stone. It may be used as a background on web pages or on some of their html elements (header, borders, menu bar, etc.). Just modify it for your needs.
Source V. Hartikainen
Prismatic Groovy Concentric Background 3 No Black
Source GDJ
Some more diagonal lines and noise, because you know you want it.
Source Atle Mo
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
Prismatic Isometric Cube Extra Pattern No Background
Source GDJ
Never out of fashion and so much hotter than the 45º everyone knows, here is a sweet 60º line pattern.
Source Atle Mo
Neat little photography icon pattern.
Source Hossam Elbialy
Prismatic Groovy Concentric Background 5
Source GDJ
ZeroCC tileable moss texture, photographed and made by me. CC0
Source Sojan Janso
Prismatic Basic Pattern 2 No Background
Source GDJ
CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
Background pattern made in "Grunge-Like" style. Available in both SVG and JPG formats. Edit to your needs then click the download button.
Source V. Hartikainen
Zero CC asphalt, pavement, texture, photographed and made by me. CC0 WARNING I FOUND A SEAM ON THIS TEXTURE
Source Sojan Janso
The classic subtle pattern. Sort of wall/brick looking. Or moon-looking?
Source Joel Klein