A grid of squares with green colours. Since the colours are randomly distributed it is automatically seamless.
Source Firkin
The rectangular tile this is based on can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin
Bigger is better, right? So here you have some large carbon fiber.
Source Factorio.us Collective
Prismatic Groovy Concentric Background 5
Source GDJ
Nice and simple crossed lines in dark gray tones.
Source Stefan Aleksić
Retro Circles Background 4 No Black
Source GDJ
Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
This is so subtle: We’re talking 1% opacity. Get your squint on!
Source Atle Mo
Light square grid pattern, great for a “DIY projects” sort of website, maybe?
Source Rafael Almeida
A seamless pattern of "sewn stripes" colored in light gray.
Source V. Hartikainen
Lovely light gray floral motif with some subtle shades.
Source GraphicsWall
To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Wild Oliva or Oliva Wilde? Darker than the others, sort of a medium dark pattern.
Source Badhon Ebrahim
One more from Badhon, sharp horizontal lines making an embossed paper feeling.
Source Badhon Ebrahim
Prismatic Curved Diamond Pattern 2 No Background
Source GDJ
A seamless pattern drawn originally in Paint.net by distorting a slice of background pattern 116 and copying the resulting triangle numerous times.
Source Firkin
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
Dark squares with some virus-looking dots in the grid.
Source Hugo Loning
If you want png files of thisu can download them here :
Source Viscious-Speed
Background Wall, Art Abstract, Block Well & CC0 texture.
Source Ractapopulous
Nicely crafted paper pattern, although a bit on the large side (500x593px).
Source Blaq Annabiosis
Horizontal and vertical lines on a light gray background.
Source Adam Anlauf
Hexagonal dark 3D pattern. What more can you ask for?
Source Norbert Levajsics