This is a hot one. Small, sharp and unique.
Source GraphicsWall
Like the name suggests, this background image consists of a pattern of dark bricks. It may be an option for you, if you are looking for something that looks like a brick wall for use as a background on web pages. It's not a masterpiece, but looks pretty nice when is tiled.
Source V. Hartikainen
Remixed from a drawing in 'The Canadian horticulturist', 1892
Source Firkin
If you like it a bit trippy, this wave pattern might be for you.
Source Ian Soper
Tiny, tiny 3D cubes. Reminds me of the good old pattern from k10k.
Source Etienne Rallion
A topographic map like this has actually been requested a few times, so here you go!
Source Sam Feyaerts
A new take on the black linen pattern. Softer this time.
Source Atle Mo
Don’t look at this one too long if you’re high on something.
Source Luuk van Baars
Crossing lines with a subtle emboss effect on a dark background.
Source Stefan Aleksić
A free seamless background pattern for use on websites.
Source V. Hartikainen
Tiny circle waves, almost like the ocean.
Source Sagive
Black brick wall pattern. Brick your site up!
Source Alex Parker
The green fibers pattern will work very well in grayscale as well.
Source Matteo Di Capua
Sharp diamond pattern. A small 24x18px tile.
Source Tom Neal
A beautiful dark padded pattern, like an old classic sofa.
Source Chris Baldie
More leather, and this time it’s bigger! You know, in case you need that.
Source Elemis
Prismatic Abstract Background Design No Black
Source GDJ
Hey, you never know when you’ll need a bird pattern, right?
Source Pete Fecteau
From a drawing in 'Sun Pictures of the Norfolk Broads', Ernest Suffling, 1892.
Source Firkin
A seamless pattern formed from a sports car on clker.com. To get the tile, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
You may use it as is, or modify it as you like.
Source V. Hartikainen
A green background pattern with warped vertical stripes and a grunge look.
Source V. Hartikainen
A pattern formed from a squared tile. The tile can be accessed in Inkscape by selecting the rectangle and using shift-alt-i.
Source Firkin
People seem to enjoy dark patterns, so here is one with some circles.
Source Atle Mo