Seamless pattern formed from a tile that can be extracted by selecting the rectangle in Inkscape and using shift-alt-i.
Source Firkin
Crossing lines with a subtle emboss effect on a dark background.
Source Stefan Aleksić
The tile this is based on can be retrieved by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
From an image on opengameart.org shared by rubberduck.
Source Firkin
Prismatic Abstract Geometric Background 2 No Black
Source GDJ
To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
From a drawing in 'Bond Slaves. The story of a struggle.', Isabella Varley, 1893.
Source Firkin
Gold Triangular Seamless Pattern No Background
Source GDJ
From a drawing in 'Uit de geschiedenis der Heilige Stede te Amsterdam', Yohannes Sterck, 1898.
Source Firkin
This one resembles a black concrete wall when is tiled. It should look great, at least with dark website themes.
Source V. Hartikainen
A pattern drawn in Paint.net and vectorized in Inkscape.
Source Firkin
A seamless web background with texture of aged grid paper.
Source V. Hartikainen
Dark, crisp and subtle. Tiny black lines on top of some noise.
Source Wilmotte Bastien
The basic shapes never get old. Simple triangle pattern.
Source Atle Mo
If you want png files of this u can download them here : viscious-speed.deviantart.com/gallery/27635117
Source Viscious-Speed
Nicely crafted paper pattern, although a bit on the large side (500x593px).
Source Blaq Annabiosis
This is a grid, only it’s noisy. You know. Reminds you of those printed grids you draw on.
Source Vectorpile
From a drawing in 'Gately's World's Progress', Charles Beale, 1886.
Source Firkin