A huge one at 800x600px. Made from a photo I took going home after work.
Source Atle Mo
CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
Same classic 45-degree pattern, dark version.
Source Luke McDonald
Vertical lines with a bumpy, yet crisp, feel to it.
Source Raasa
Sharp pixel pattern looking like some sort of fabric.
Source Dmitry
Remixed from a drawing in 'Maidenhood; or, the Verge of the Stream', Laura Jewry, 1876.
Source Firkin
Pixel by pixel, sharp and clean. Very light pattern with clear lines.
Source M.Ashok
Formed from a tile based on a drawing from 'Viaggi d'un artista nell'America Meridionale', Guido Boggiani, 1895.
Source Firkin
A seamless pattern the unit cell for which can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
This is a remix of "blue wave-seigaiha".I hope this subtle color version of Seigaiha would be suitable for background .
Source Yamachem
Looks a bit like concrete with subtle specks spread around the pattern.
Source Mladjan Antic
From a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
If you want png files of thisu can download them here :
Source Viscious-Speed
Never out of fashion and so much hotter than the 45º everyone knows, here is a sweet 60º line pattern.
Source Atle Mo
Used correctly, this could be nice. Used in a bad way, all hell will break loose.
Source Atle Mo
One more sharp little tile for you. Subtle circles this time.
Source Blunia
A free black metallic background pattern. Here's a new pattern I made that looks metallic.
Source V. Hartikainen
From a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
To get the tile this is based on, select the rectangle in Inkscape and use shift-alt-i.
Source Firkin
Subtle scratches on a light gray background.
Source Andrey Ovcharov
A seamless pattern from a tile drawn in Paint.net and vectorised in Vector Magic
Source Firkin
An abstract pale yellow paper-like background with stains colored in yellow and green.
Source V. Hartikainen
Looks as if it's spray painted on the wall. You can be sure that this pattern will seamlessly fill your backgrounds on web pages.
Source V. Hartikainen