Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
The tile can be had by using shift+alt+i on the selected rectangle in Inkscape
Source Firkin
Scanned some rice paper and tiled it up for you. Enjoy.
Source Atle Mo
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
Sort of reminds me of those old house wallpapers.
Source Tish
Snap! It’s a pattern, and it’s not grayscale! Of course you can always change the color in Photoshop.
Source Atle Mo
Prismatic Abstract Background Design
Source GDJ
Medium gray fabric pattern with 45-degree lines going across.
Source Atle Mo
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 'Art Embroidery', M.S. Lockwood and E. Glaister, 1878.
Source Firkin
This is the remix of "blue wave-seigaiha".The image depicts a seamless pattern of the front upper part of Japanese five yen coin which is used currently.This design represents a rice with ripe golden ears.
Source Yamachem
ZeroCC tileable moss texture, photographed and made by me. CC0
Source Sojan Janso
Light and tiny, just the way you like it.
Source Rohit Arun Rao
Number 3 in a series of 5 beautiful patterns. Can be found in colors on the submitter’s website.
Source Janos Koos
Beautiful dark noise pattern with some dust and grunge.
Source Vincent Klaiber
To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
This background pattern contains a seamless texture of bark. It's not very realistic, but I think it looks quite nice.
Source V. Hartikainen
This seamless light brown background texture resembles a wallpaper with vertical stripes. One way to use it is as a tiled background on web sites.
Source V. Hartikainen
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
An interesting dark spotted pattern at an angle.
Source Hendrik Lammers
I asked Gjermund if he could make a pattern for us – result!
Source Gjermund Gustavsen
Prismatic Abstract Line Art Pattern Background 2
Source GDJ
Prismatic Snowflakes Pattern 2 No Background
Source GDJ
The image depicts a seamless pattern which includes hexagonally-aligned gourds with BG in light-brown.
Source Yamachem