Green Web Background, Seamless tile.
Source V. Hartikainen
This one takes you back to math class. Classic mathematic board underlay.
Source Josh Green
To get the repeating unit, select the rectangle in Inkscape and use shift+alt+i
Source Firkin
Horizontal and vertical lines on a light gray background.
Source Adam Anlauf
A white version of the very popular linen pattern.
Source Ant Ekşiler
Remixed from a drawing in 'Canadian forest industries July-December', 1915
Source Firkin
Bigger is better, right? So here you have some large carbon fiber.
Source Factorio.us Collective
Abstract Tiled Background Extended 12
Source GDJ
A seamless pattern formed from miutopia mug remixes on a tablecloth.
Source Firkin
From a drawing in 'Bond Slaves. The story of a struggle.', Isabella Varley, 1893.
Source Firkin
This is sort of fresh, but still feels a bit old school.
Source Martuchox
Adapted from an image that was uploaded to Pixabay by Anerma.
Source Firkin
A seamless pattern formed from a sports car on clker.com. To get the tile, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Tiny little flowers growing on your screen. Nice, huh?
Source Themes Tube
Abstract Tiled Background Extended 8
Source GDJ
Wasn't satisfied with the original's colouring. Too much component transfer and colormatrixes yet the results are lacking a bit. So this time it is a simple black to transparent fade, making it possible remixing easily once there will be other blending modes supported as well. Probably in inkscape 0.92.
Source Lazur URH
Some rectangles, a bit of dust and grunge, plus a hint of concrete.
Source Atle Mo
Prismatic Geometric Tessellation Pattern 4 No Background
Source GDJ
Some more diagonal lines and noise, because you know you want it.
Source Atle Mo
Sort of like the Photoshop transparent background, but better!
Source Alex Parker
Prismatic Groovy Concentric Background
Source GDJ
Prismatic Abstract Background Design No Black
Source GDJ
A seamless pattern the starting point for which was a 'light rays' rendering in Paint.net.
Source Firkin