Seamless pattern the basic tile for which can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Seamless Green Tile Background
Source V. Hartikainen
Fix and cc0 to get the tile this is based on.
Source SliverKnight
Remixed from a PNG that was uploaded to Pixabay by VictorianLady
Source Firkin
The image depicts meshed silhouettes of various things.The original image is an OCAL clipart called "Enter FOSSASIA 2016 #IoT T-shirt Design Contest" uploaded by "openclipart".Thanks.
Source Yamachem
Just what the name says, paper fibers. Always good to have.
Source Heliodor jalba
From a drawing in 'Sun Pictures of the Norfolk Broads', Ernest Suffling, 1892.
Source Firkin
This is a remix of "blue wave-seigaiha".I hope this subtle color version of Seigaiha would be suitable for background .
Source Yamachem
Sharp pixel pattern looking like some sort of fabric.
Source Dmitry
A pattern formed from a photograph of a 16th century ceramic tile.
Source Firkin
Looks like a technical drawing board: small squares forming a nice grid.
Source We Are Pixel8
A heavy hitter at 400x400px, but lovely still.
Source Breezi
Prismatic Rounded Squares Grid 3 No Background
Source GDJ
A cute x, if you need that sort of thing.
Source Juan Scrocchi
This is sort of fresh, but still feels a bit old school.
Source Martuchox
Vertical lines with a bumpy, yet crisp, feel to it.
Source Raasa
Light and tiny, just the way you like it.
Source Rohit Arun Rao
Derived from a corner decoration itself found as a jpg on Pixabay.
Source Firkin
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
Sharp pixel pattern, just like the good old days.
Source Paridhi