Produced using the clouds, flames and glass blocks plug-ins in Paint.net and the resulting .PNG vectorised with Vector Magic.
Source Firkin
With a name this awesome, how can I go wrong?
Source Nikolay Boltachev
Crossing lines with a subtle emboss effect on a dark background.
Source Stefan Aleksić
This background image has seamless texture that resembles a surface of gray stone.
Source V. Hartikainen
A seamless green background texture. The image is distributed under a Creative Commons License (like all of the images here).
Source V. Hartikainen
The unit cell for this seamless pattern can be had in Inkscape by selecting the rectangle and using shift+alt+i.
Source Firkin
Background formed from the iconic plastic construction bricks that gave me endless hours of fun when I was a lad.
Source Firkin
From a drawing in 'Navigations de Alouys de Cademoste.-La Navigation du Capitaine Pierre Sintre', Alvise da ca da Mosto, 1895.
Source Firkin
This one needs to be used in small areas; you can see it repeat.
Source Luca
Green Background Pattern
Source V. Hartikainen
Vector version of a png that was uploaded to Pixabay by pencilparker
Source Firkin
A seamless pattern based on a rectangular tile that can be retrieved in Inkscape by selecting the rectangle and using shift-alt-i.
Source Firkin
Prismatic Snowflakes Pattern 3 No Background
Source GDJ
Adapted from a drawing in 'Line and form', Walter Crane, 1914.
Source Firkin
Actually remixed from a pattern on Pixabay. But then noticed a very similar one on Openclipart.org uploaded by btj51q2.
Source Firkin
You were craving more leather, so I whipped this up by scanning a leather jacket.
Source Atle Mo
Coming in at 666x666px, this is an evil big pattern, but nice and soft at the same time.
Source Atle Mo
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
A bit like smudged paint or some sort of steel, here is scribble light.
Source Tegan Male
Heavy depth and shadows here, but might work well on some mobile apps.
Source Damian Rivas
This makes me wanna shoot some pool! Sweet green pool table pattern.
Source Caveman
Just to prove my point, here is a slightly modified dark version.
Source Atle Mo
Inspired by this, I came up with this pattern. Madness!
Source Atle Mo
Super subtle indeed, a medium gray pattern with tiny dots in a grid.
Source Designova
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