This could be a hippy vintage wallpaper.
Source Tileable Patterns
Some more diagonal lines and noise, because you know you want it.
Source Atle Mo
A chequerboard pattern with a fruit theme. The fruits are from a posting by inkscapeforum.it.
Source Firkin
Dark blue concrete wall with some small dust spots.
Source Atle Mo
A brown seamless wood texture in a form of stripe pattern. The result has turned out pretty well, in my opinion.
Source V. Hartikainen
You don’t see many mid-tone patterns here, but this one is nice.
Source Joel Klein
Prismatic Curved Diamond Pattern 7 No Background
Source GDJ
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
Clean and crisp lines all over the place. Wrap it up with this one.
Source Dax Kieran
The green fibers pattern will work very well in grayscale as well.
Source Matteo Di Capua
Seamless pattern the tile for which can be had by using shift-alt-I on the selected rectangle in Inkscape.
Source Firkin
From a drawing in 'Uit de geschiedenis der Heilige Stede te Amsterdam', Yohannes Sterck, 1898.
Source Firkin
People seem to enjoy dark patterns, so here is one with some circles.
Source Atle Mo
Actually remixed from a pattern on Pixabay. But then noticed a very similar one on Openclipart.org uploaded by btj51q2.
Source Firkin
Seamless Prismatic Geometric Pattern With Background
Source GDJ
A monochrome pattern from a tile that can be had by selecting the rectangle in Inkscaope and using shift+alt+i
Source Firkin
From a drawing in 'Art Embroidery', M.S. Lockwood and E. Glaister, 1878.
Source Firkin
The classic notebook paper with horizontal stripes.
Source Are Sundnes
A free green background pattern with a pattern of rhombuses on a seamless texture. Feel free to use it as a tiled background image on your web site.
Source V. Hartikainen
Gold Triangular Seamless Pattern No Background
Source GDJ
With a name like this, it has to be hot. Diagonal lines in light shades.
Source Isaac
You can never get enough of these tiny pixel patterns with sharp lines.
Source Designova