Simple gray checkered lines, in light tones.
Source Radosław Rzepecki
Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
f you want png files of this u can download them here : viscious-speed.deviantart.com/gallery/27635117
Source Viscious-Speed
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
A comeback for you: the popular Escheresque, now in black.
Source Patten
One more from Badhon, sharp horizontal lines making an embossed paper feeling.
Source Badhon Ebrahim
Vector version of a png that was uploaded to Pixabay by pencilparker
Source Firkin
To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
The image depicts a seamless pattern of a Japanese family crest called "chidori" in Japanese .A chidori in Japanese means a plover in English.
Source Yamachem
The first pattern on here using opacity. Try it on a site with a colored background, or even using mixed colors.
Source Nathan Spady
Textured Red Brown Plastic, Free Background Pattern. Although there's already enough plastic in our lives, let's bring it to the web too.)
Source V. Hartikainen
The image depicts a seamless pattern made using a bird's face.
Source Yamachem
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
Seamless pattern formed from a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin
Remixed from a drawing in 'Maidenhood; or, the Verge of the Stream', Laura Jewry, 1876.
Source Firkin
A seamless pattern with wide vertical stripes colored in pale yellow.
Source V. Hartikainen
A free seamless background image with abstract texture of green "curtain".
Source V. Hartikainen
Prismatic Groovy Concentric Background 4
Source GDJ
This reminds me of Game Cube. A nice light 3D cube pattern.
Source Sander Ottens
This one could be the shirt of a golf player. Angled lines in different thicknesses.
Source Olivier Pineda
The tile this is based on can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin
A pattern derived from repeating unit cells each derived from part of a fractal rendering in paint.net.
Source Firkin
A lovely light gray pattern with stripes and a dash of noise.
Source V. Hartikainen
A seamless pattern the unit cell for which can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin