A seamless chequerboard pattern formed from a tile that can be had in Inkscape by selecting the rectangle and using shift+alt+i. Alternative colour scheme.
Source Firkin
From a drawing in 'The Quiver of Love', Walter Crane, 1876
Source Firkin
To get the tile this is based on, select the rectangle in Inkscape and use shift-alt-i.
Source Firkin
From a drawing in 'Sun Pictures of the Norfolk Broads', Ernest Suffling, 1892.
Source Firkin
CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
An abstract Background pattern of purple twisty patterns.
Source TikiGiki
Don’t look at this one too long if you’re high on something.
Source Luuk van Baars
Actually, there's no clouds in it, but I think it looks quite nice.
Source V. Hartikainen
Prismatic Groovy Concentric Background No Black
Source GDJ
Made by distorting a simple pattern using the 'sin waves' plugin for Paint.net and vectorising in Vector Magic
Source Firkin
Lovely light gray floral motif with some subtle shades.
Source GraphicsWall
A free web background image with a seamless concrete-like texture and an Indian-red color.
Source V. Hartikainen