Simple wide squares with a small indent. Fits all.
Source Petr Šulc.
A background pattern with a look of rough fabric.
Source V. Hartikainen
Prismatic Groovy Concentric Background 2 No Black
Source GDJ
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
Dark squares with some virus-looking dots in the grid.
Source Hugo Loning
Dark wooden pattern, given the subtle treatment. based on texture from Cloaks. https://cloaks.deviantart.com
Source Atle Mo
This one takes you back to math class. Classic mathematic board underlay.
Source Josh Green
The unit cell for this seamless pattern can be had in Inkscape by selecting the rectangle and using shift+alt+i
Source Firkin
Zero CC Mossy stone tileable texture, photographed and made by me. CC0
Source Sojan Janso
A free seamless background image with a texture of dark red "canvas". It should look very nice on web sites.
Source V. Hartikainen
A pattern formed from a photograph of a 16th century ceramic tile.
Source Firkin
A new take on the black linen pattern. Softer this time.
Source Atle Mo
Not the Rebel alliance, but a dark textured pattern.
Source Hendrik Lammers
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
Got some felt in my mailbox today, so I scanned it for you to use.
Source Atle Mo
Prismatic Geometric Tessellation Pattern 2 No Background
Source GDJ
A slightly grainy paper pattern with small horizontal and vertical strokes.
Source Atle Mo
Found on the ground in french cafe in kunming, Yunnan, china
Source Rejon
From a drawing in 'Art Embroidery', M.S. Lockwood and E. Glaister, 1878.
Source Firkin