Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
This one is amazing, truly original. Go use it!
Source Viahorizon
Sometimes simple really is what you need, and this could fit you well.
Source Factorio.us Collective
This one resembles a black concrete wall when is tiled. It should look great, at least with dark website themes.
Source V. Hartikainen
An aged paper background tile with smeared and pressed text.
Source V. Hartikainen
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
An alternative colour scheme for the original seamless texture formed from an image on Pixabay.
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
To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Derived from an image that was uploaded to Pixabay by mdmelo.
Source Firkin
A seamless pattern formed from a square tile. The tile can be retrieved by selecting the rectangle in Inkscape and using shift-alt-I. Version with black background.
Source Firkin
Inspired by this, I came up with this pattern. Madness!
Source Atle Mo
Classic 45-degree pattern, light version.
Source Luke McDonald
Classy golf-pants pattern, or crossed stripes if you will.
Source Will Monson
Prismatic Isometric Cube Wireframe Pattern No Background
Source GDJ
From a drawing in 'In an Enchanted Island', William Mallock, 1892.
Source Firkin
Retro Circles Background 4 No Black
Source GDJ
A large (588x375px) sand-colored pattern for your ever-growing collection. Shrink at will.
Source Alex Tapein
Your eyes can trip a bit from looking at this – use it wisely.
Source Michal Chovanec
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
New paper pattern with a slightly organic feel to it, using some thin threads.
Source Atle Mo
To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin