Sometimes simple really is what you need, and this could fit you well.
Source Factorio.us Collective
Derived from an image that was uploaded to Pixabay by starchim01
Source Firkin
Remixed from a drawing in 'Chambéry à la fin du XIVe siècle', Timoleon Chapperon, 1863.
Source Firkin
Have you wondered about how it feels to be buried alive? Here is the pattern for it.
Source Hendrik Lammers
Remixed from an image that was uploaded to Pixabay by KirstenStar
Source Firkin
A large (588x375px) sand-colored pattern for your ever-growing collection. Shrink at will.
Source Alex Tapein
Fabric-ish patterns are close to my heart. French Stucco to the rescue.
Source Christopher Buecheler
From a drawing in 'Resa i Afrika, genom Angola, Ovampo och Damaraland', P. Moller, 1899.
Source Firkin
Prismatic Abstract Geometric Background 5 No Black
Source GDJ
An interesting dark spotted pattern at an angle.
Source Hendrik Lammers
Very dark pattern with some noise and 45-degree lines.
Source Stefan Aleksić
A bit like smudged paint or some sort of steel, here is scribble light.
Source Tegan Male
A light gray wall or floor (you decide) of concrete.
Source Atle Mo
Remixed from a drawing in 'Line and form", Walter Crane, 1914.
Source Firkin
Heavy depth and shadows here, but might work well on some mobile apps.
Source Damian Rivas
To get the repeating unit, select the rectangle in Inkscape and use shift+alt+i
Source Firkin
Remixed from a drawing in 'A Child of the Age', Francis Adams, 1894.
Source Firkin
From a drawing in 'Art Embroidery', M.S. Lockwood and E. Glaister, 1878.
Source Firkin
Everyone loves a diamond, right? Make your site sparkle.
Source AJ Troxell
Prismatic Curved Diamond Pattern 6 No Background
Source GDJ
This seamless background image should look nice on websites. It has a dark blue gray texture with vertical stripes, it tiles seamlessly and, like all of the background images here, it's free. So, if you like it, take it!
Source V. Hartikainen
More carbon fiber for your collections. This time in white or semi-dark gray.
Source Badhon Ebrahim
In the spirit of WWDC 2011, here is a dark iOS inspired linen pattern.
Source Atle Mo
This is so subtle I hope you can see it! Tweak at will.
Source Alexandre Naud