Seamless pattern the tile for which can be had by using shift-alt-I on the selected rectangle in Inkscape.
Source Firkin
One can never have too few rice paper patterns, so here is one more.
Source Atle Mo
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
A seamlessly tile-able grunge background image.
Source V. Hartikainen
An attempt for cleaning up the original image in a few steps.
Source Lazur URH
Looks like a technical drawing board: small squares forming a nice grid.
Source We Are Pixel8
A bit of scratched up grayness. Always good.
Source Dmitry
Prismatic 3D Isometric Tessellation Pattern 6
Source GDJ
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
Not sure if this is related to the Nami you get in Google image search, but hey, it’s nice!
Source Dertig Media
Prismatic Geometric Pattern Background No Black
Source GDJ
This was formed by distorting an image of a background on Pixabay.
Source Firkin
To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Remixed from a drawing in 'Line and form", Walter Crane, 1914.
Source Firkin
Remixed from an image that was uploaded to Pixabay by Pixeline
Source Firkin
Background Wall, Art Abstract, Block Well & CC0 texture.
Source Ractapopulous
From a drawing in 'Sun Pictures of the Norfolk Broads', Ernest Suffling, 1892.
Source Firkin
Feel free to use this seamless background texture as a background on a web site. It's colored in a light pink color and is seamlessly tile-able.
Source V. Hartikainen
This light blue background pattern is quite pleasing to the eye, it consists of a tiny rough grid pattern, which is seamless by design. That's it, if you like the color, you can use this seamless pattern in a web design without making any further modifications to it.
Source V. Hartikainen
Dare I call this a «flat pattern»? Probably not.
Source Dax Kieran
Here's a seamless brown cork board background texture. Feel free to download or reshare if you like.
Source V. Hartikainen