I took the liberty of using Dmitry’s pattern and made a version without perforation.
Source Atle Mo
This was formed by distorting an image of a background on Pixabay.
Source Firkin
To get the tile this is formed from select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
This one takes you back to math class. Classic mathematic board underlay.
Source Josh Green
Prismatic Abstract Geometric Background 4 No Black
Source GDJ
To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Uses spirals from Pixabay. To get the basic tile select the rectangle in Inkscape and use shift+alt+i
Source Firkin
Some rectangles, a bit of dust and grunge, plus a hint of concrete.
Source Atle Mo
Remixed from a drawing in 'The March of Loyalty', Letitia MacClintock, 1884.
Source Firkin
Remixed from a design seen on Pixabay. The basic tile can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Floral patterns will never go out of style, so enjoy this one.
Source Lasma
To get the repeating unit, select the rectangle in Inkscape and use shift+alt+i
Source Firkin
Embossed lines and squares with subtle highlights.
Source Alex Parker
Zero CC tileable ground (#2) cracked, crackled texture, made by me. CC0
Source Sojan Janso
Alternative colour scheme for the original floral pattern.
Source Firkin
People seem to enjoy dark patterns, so here is one with some circles.
Source Atle Mo
A seamless pattern the unit cell for which can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
You know you love wood patterns, so here’s one more.
Source Richard Tabor
Light honeycomb pattern made up of the classic hexagon shape.
Source Federica Pelzel
This light yellow background pattern consists of an irregular pattern of spots. Here's a light background pattern with yellowish tint.
Source V. Hartikainen
Very dark pattern with some noise and 45-degree lines.
Source Stefan Aleksić
Your eyes can trip a bit from looking at this – use it wisely.
Source Michal Chovanec