Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
A background pattern with wavy green vertical stripes. This one has green stripes on a white background. Download if you like it.
Source V. Hartikainen
To get the repeating unit, select the rectangle in Inkscape and use shift+alt+i
Source Firkin
Here's a bluish gray striped background pattern for use on web sites.
Source V. Hartikainen
To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Prismatic Curved Diamond Pattern 3 No Background
Source GDJ
Drawn in Paint.net and vectorised in Vector Magic.
Source Firkin
A rusty grunge background for websites. Feel free to use it in your site's theme.
Source V. Hartikainen
Simple combination of stripy squares with their negatively coloured counterparts
Source Firkin
Heavy depth and shadows here, but might work well on some mobile apps.
Source Damian Rivas
Dead simple but beautiful horizontal line pattern.
Source Fabian Schultz
I asked Gjermund if he could make a pattern for us – result!
Source Gjermund Gustavsen
This pack of filters can help you adding a blocky overlay to objects. May come handy at drawing blocks of stone.
Source Lazur URH
A topographic map like this has actually been requested a few times, so here you go!
Source Sam Feyaerts
Prismatic Hypnotic Pattern No Background
Source GDJ
Washi (和紙?) is a type of paper made in Japan. Here’s the pattern for you!
Source Carolynne
This is a remix of "flower seamless pattern".I rotated the original image by 90 degrees.This is a seamless pattern of flowers.These horizontal wavy lines are one of Edo patterns which is called "tatewaku or tachiwaku or 立湧" that represents uprising steam or vapor.
Source Yamachem
Paper model of a tetrahedron. Modelo de papel de um tetraedro.
Source laobc
I’m guessing this is related to the Sony Vaio? It’s a nice pattern no matter where it’s from.
Source Zigzain
Seamless , tileable CC-0 texture. Created by my own, feel free to use wherever you want!
Source Linolafett
A subtle shadowed checkered pattern. Increase the lightness for even more subtle sexiness.
Source Josh Green
A colourful background drawn originally in paint.net and vectorised in Vector Magic.
Source Firkin