Horizontal and vertical lines on a light gray background.
Source Adam Anlauf
Subtle scratches on a light gray background.
Source Andrey Ovcharov
A set of paper filters. The base texture is generated the same way, only the compositing mode is varied.
Source Lazur URH
Some more diagonal lines and noise, because you know you want it.
Source Atle Mo
The tile can be had by using shift+alt+i on the selected rectangle in Inkscape
Source Firkin
Beautiful dark noise pattern with some dust and grunge.
Source Vincent Klaiber
A textured orange background pattern with vertical stripes.
Source V. Hartikainen
A free seamless background texture of "timber wall" (colored in dark brown).
Source V. Hartikainen
Prismatic Curved Diamond Pattern 7 No Background
Source GDJ
Here's a camo print with more tan and less green, such as might be used in a desert scenario. This is tileable, so it can be used as a wallpaper or background.
Source Eady
Little x’es, noise and all the stuff you like. Dark like a Monday, with a hint of blue.
Source Tom McArdle
A seamless pattern formed from background pattern 102
Source Firkin
Sometimes simple really is what you need, and this could fit you well.
Source Factorio.us Collective
A slightly grainy paper pattern with small horizontal and vertical strokes.
Source Atle Mo
A seamless pattern the starting point for which was a 'colour modulo' texture in Paint.net.
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
Hexagonal dark 3D pattern. What more can you ask for?
Source Norbert Levajsics
A topographic map like this has actually been requested a few times, so here you go!
Source Sam Feyaerts
A seamless textured paper for backgrounds. Colored in pale orange hues.
Source V. Hartikainen
Prismatic Curved Diamond Pattern 2 No Background
Source GDJ
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
A monochrome pattern from a tile that can be had by selecting the rectangle in Inkscaope and using shift+alt+i
Source Firkin