CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
Remixed from a drawing in 'Line and form', Walter Crane, 1914.
Source Firkin
The green fibers pattern will work very well in grayscale as well.
Source Matteo Di Capua
Same as gray sand but lighter. A sandy pattern with small light dots, and some angled strokes.
Source Atle Mo
Pixel by pixel, sharp and clean. Very light pattern with clear lines.
Source M.Ashok
Design drawn in Paint.net, vectorised using Vector Magic and finished in Inkscape.
Source Firkin
A mid-tone gray pattern with some cement looking texture.
Source Hendrik Lammers
One more sharp little tile for you. Subtle circles this time.
Source Blunia
Bumps, highlight and shadows – all good things.
Source Badhon Ebrahim
Lovely pattern with splattered vintage speckles.
Source David Pomfret
A seamless pattern from a tile made from a jpg on Pixabay. To get the tile select the rectangle in Inkscape and use shift-alt-i.
Source Firkin
Remixed from a drawing in 'Line and form", Walter Crane, 1914.
Source Firkin
This background pattern looks like bamboo to me. Feel free to download it for your website (for your blog perhaps?).
Source V. Hartikainen
Sharp pixel pattern looking like some sort of fabric.
Source Dmitry
A grid of squares with green colours. Since the colours are randomly distributed it is automatically seamless.
Source Firkin
I took the liberty of using Dmitry’s pattern and made a version without perforation.
Source Atle Mo
The original enhanced with some gradients.
Source Firkin
Drawn in Paint.net using the kaleidoscope plug-in and vectorised.
Source Firkin
This pattern comes in orange, and it looks as if it is "made of glass".
Source V. Hartikainen
A free tileable background colored in off-white (antique white) color.
Source V. Hartikainen
Dead simple but beautiful horizontal line pattern.
Source Fabian Schultz
A seamless pattern from a tile made from a jpg on Pixabay. To get the tile select the rectangle in Inkscape and use shift-alt-i.
Source Firkin
The act or state of corrugating or of being corrugated, a wrinkle; fold; furrow; ridge.
Source Anna Litvinuk
Colorful Floral Background No Black
Source GDJ