A seamless pattern the starting point for which was a 'rainbow twist' texture in Paint.net.
Source Firkin
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
Prismatic Geometric Pattern Background
Source GDJ
From a drawing in 'Kingsdene', Maria Fetherstonehaugh, 1878.
Source Firkin
A nice and simple white rotated tile pattern.
Source Another One
Plywood Web Background background image for use in web design.
Source V. Hartikainen
From a drawing in 'Les Chroniqueurs de l'Histoire de France depuis les origines jusqu'au XVIe siècle', Henriette Witt, 1884.
Source Firkin
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
A pattern formed from a photograph of a 16th century ceramic tile.
Source Firkin
The image depicts a Japanese Edo pattern called "kanoko or 鹿の子" meaning "fawn" which has a fur with small white spots.
Source Yamachem
Seamless pattern the tile for which can be had by using shift-alt-I on the selected rectangle in Inkscape.
Source Firkin
Here's a bluish gray striped background pattern for use on web sites.
Source V. Hartikainen
From a drawing in 'An Old Maid's Love. A Dutch tale told in English', Maarten Maartens, 1891.
Source Firkin
From a drawing in 'Studies for Stories', Jean Ingelow, 1864.
Source Firkin
A rusty grunge background for websites. Feel free to use it in your site's theme.
Source V. Hartikainen
Prismatic 3D Isometric Tessellation Pattern 6
Source GDJ
Prismatic Geometric Tessellation Pattern 2 No Background
Source GDJ
From a drawing in 'Uit de geschiedenis der Heilige Stede te Amsterdam', Yohannes Sterck, 1898.
Source Firkin
Prismatic Geometric Tessellation Pattern 4 No Background
Source GDJ
It almost looks a bit blurry, but then again, so are fishes.
Source Petr Šulc
From a drawing in 'Art Embroidery', M.S. Lockwood and E. Glaister, 1878.
Source Firkin
Prismatic Groovy Concentric Background 2 No Black
Source GDJ
And some more testing, this time with Seamless Studio. It’s Robots FFS!
Source Seamless Studio