A seamless green background texture. The image is distributed under a Creative Commons License (like all of the images here).
Source V. Hartikainen
Just to prove my point, here is a slightly modified dark version.
Source Atle Mo
A seamless web texture with illustration of pale color stains on canvas.
Source V. Hartikainen
A seamless pattern that includes the original tile (go to Objects / Pattern / Pattern To Objects in Inkscape's menu to extract it).
Source GDJ
Zero CC tileable moss or lichen covered stone texture, edited from pixabay. CC0
Source Sojan Janso
Remixed from a drawing in 'Some account of the Worshipful Company of Ironmongers', John Nicholl, 1866.
Source Firkin
This was formed by distorting an image of a background on Pixabay.
Source Firkin
An interesting dark spotted pattern at an angle.
Source Hendrik Lammers
A free seamless background image with a texture of dark red "canvas". It should look very nice 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
A seamless texture of black leather. I think it will look best when used in headers, footers or sidebars.
Source V. Hartikainen
From a drawing in 'Uit de geschiedenis der Heilige Stede te Amsterdam', Yohannes Sterck, 1898.
Source Firkin
More in the paper realm, this time with fibers.
Source Jorge Fuentes
Derived from an image that was uploaded to Pixabay by Kaz
Source Firkin
Submitted in a cream color, but you know how I like it.
Source Devin Holmes
The image is a seamless pattern which is derived from a vine .Consequently, the vine got like dots via vectorization.The original vine is here:jp.pinterest.com/pin/500744052301410188/
Source Yamachem
There are many carbon patterns, but this one is tiny.
Source Designova
Super dark, crisp and detailed. And a Kill Bill reference.
Source Badhon Ebrahim
Inspired by a 1930s wallpaper pattern I saw on TV.
Source Firkin
A free web background image with a seamless concrete-like texture and an Indian-red color.
Source V. Hartikainen
From a drawing in 'Sun Pictures of the Norfolk Broads', Ernest Suffling, 1892.
Source Firkin
Remixed from a drawing in 'Line and form', Walter Crane, 1914.
Source Firkin
A grid of squares with green colours. Since the colours are randomly distributed it is automatically seamless.
Source Firkin
I’m starting to think I have a concrete wall fetish.
Source Atle Mo
Remixed from a drawing in 'Maidenhood; or, the Verge of the Stream', Laura Jewry, 1876.
Source Firkin