Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
Prismatic Groovy Concentric Background 2
Source GDJ
Alternative colour scheme for the original floral pattern.
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
From a drawing in 'Cassell's Library of English Literature', Henry Morley, 1883.
Source Firkin
Could be paper, could be a Polaroid frame – up to you!
Source Chaos
On a large canvas you can see it tiling, but used on smaller areas, it’s beautiful.
Source Paul Phönixweiß
From a drawing in 'Line and form', Walter Crane, 1914.
Source Firkin
From a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
This one is amazing, truly original. Go use it!
Source Viahorizon
Seamless pattern the tile for which can be had by using shift-alt-I on the selected rectangle in Inkscape.
Source Firkin
From a drawing in 'Sun Pictures of the Norfolk Broads', Ernest Suffling, 1892.
Source Firkin
Alternative colour scheme for the original floral pattern.
Source Firkin
This was formed by distorting an image of a background on Pixabay.
Source Firkin
A seamless background drawn in Paint.net and vectorised with Vector Magic. The starting point was a photograph of drinking straws from Pixabay.
Source Firkin
A seamless pattern formed from background pattern 102
Source Firkin
Used the 6th circle pattern designed by Viscious-Speed to create a print that can be used for card making or scrapbooking. Save as a PDF file for the best printing option.
Source Lovinglf
Simple wide squares with a small indent. Fits all.
Source Petr Šulc.
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
Abstract Ellipses Background Grayscale
Source GDJ
Inspired by this, I came up with this pattern. Madness!
Source Atle Mo
Prismatic Groovy Concentric Background 4
Source GDJ
This light background pattern has a texture of "frozen" surface with diagonal stripes. Here's an yet another addition to the collection of free website backgrounds.
Source V. Hartikainen