Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
From a drawing in 'At home', J. Sowerby, J. Crane and T. Frederick, 1881.
Source Firkin
The image depicts a seamless pattern of a fishnet with a plenty of fish.It may be a lucky charm for fishermen.
Source Yamachem
Detailed but still subtle and quite original. Lovely gray shades.
Source Kim Ruddock
Here's a brown background pattern with subtle stripes. I hope you'll like the color. If not, feel free to change it using an image editor, if you know how of course. Personally, I'm using GIMP to create these backgrounds.
Source V. Hartikainen
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 'Chambéry à la fin du XIVe siècle', Timoleon Chapperon, 1863.
Source Firkin
From a drawing in 'Hubert Montreuil, or the Huguenot and the Dragoon', Francisca Ouvry, 1873.
Source Firkin
From a design found in 'History of the Virginia Company of London; with letters to and from the first Colony, never before printed', Edward Neill, 1869.
Source Firkin
Same classic 45-degree pattern, dark version.
Source Luke McDonald
Looks as if it's spray painted on the wall. You can be sure that this pattern will seamlessly fill your backgrounds on web pages.
Source V. Hartikainen
Drawn in Paint.net and vectorised in Vector Magic.
Source Firkin
A comeback for you: the popular Escheresque, now in black.
Source Patten
Zero CC tileable brick texture, photographed and made by me. CC0
Source Sojan Janso
The image is the remix of "wire-mesh fence seamless pattern" .This is a more minute version of it.Sorry for the file size.Using path>difference in Inkscape, I will cut out any silhouette from this pattern and create a "meshed silhouette".
Source Yamachem
Seamless Background For Websites. It has a texture similar to cork-board.
Source V. Hartikainen
Seamless , tileable CC-0 texture. Created by my own, feel free to use wherever you want!
Source Linolafett
Looks like an old wall. I guess that’s it then?
Source Viahorizon
Inspired by a pattern found in 'Sun Pictures of the Norfolk Broads', Ernest Suffling, 1892.
Source Firkin
Drawn in Paint.net and vectorised in Vector Magic.
Source Firkin
A bit strange this one, but nice at the same time.
Source Diogo Silva
Colourful background achieved with gradient fills.
Source Firkin
Got some felt in my mailbox today, so I scanned it for you to use.
Source Atle Mo
Utilising some flowers from Almeidah. To get the unit tile, select the rectangle in Inkscape and use shift-alt-i.
Source Firkin
Prismatic Groovy Concentric Background
Source GDJ
Prismatic Geometric Tessellation Pattern 4 No Background
Source GDJ