A good starting point for a cardboard pattern. This would work well in a variety of colors.
Source Atle Mo
Original minus the background
Source Firkin
Here's a brown background pattern with subtle stripes. I hope you'll like the color. If not, feel free to change it using an image editor, if you know how of course. Personally, I'm using GIMP to create these backgrounds.
Source V. Hartikainen
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
A seamless background texture of old cardboard.
Source V. Hartikainen
To get the tile this is based on, select the rectangle in Inkscape and use shift-alt-i.
Source Firkin
The green fibers pattern will work very well in grayscale as well.
Source Matteo Di Capua
A grayscale fabric pattern with vertical lines of stitch holes.
Source V. Hartikainen
Prismatic Geometric Pattern Background
Source GDJ
A seamless pattern the unit cell for which can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin
A seamless pattern the starting point for which was a 'rainbow twist' texture in Paint.net.
Source Firkin
A version without colours blended together to give a different look.
Source Firkin
Clover with background for St. Patrick's Day. Add to a card with a doily, ribbon, a leprechaun or other embellishments.
Source BAJ
This background image is great for using in web design or graphic design projects. And don't forget to visit the homepage. I frequently update this resource with fresh tileable backgrounds.
Source V. Hartikainen
A series of 5 patterns. That’s what the P stands for, if you didn’t guess it.
Source Dima Shiper
Detailed but still subtle and quite original. Lovely gray shades.
Source Kim Ruddock
Prismatic Isometric Cube Extra Pattern No Background
Source GDJ
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
The tile this fill pattern is based on can be had by using shift+alt+i on the rectangle.
Source Firkin
I guess this one is inspired by an office. A dark office.
Source Andrés Rigo.
The basic shapes never get old. Simple triangle pattern.
Source Atle Mo
Could remind you a bit of those squares in Super Mario Bros, yeh?
Source Jeff Wall
Seamless pattern the tile for which can be had by using shift-alt-I on the selected rectangle in Inkscape.
Source Firkin
Fake or not, it’s quite luxurious.
Source Factorio.us Collective
A seamless pattern based on a square tile that can be retrieved in Inkscape by selecting the rectangle and using shift-alt-i.
Source Firkin