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
Prismatic Curved Diamond Pattern 3 No Background
Source GDJ
This was formed by distorting an image of a background on Pixabay.
Source Firkin
Abstract Ellipses Background Grayscale
Source GDJ
With a name this awesome, how can I go wrong?
Source Nikolay Boltachev
Retro Circles Background 8 No Black
Source GDJ
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
Seamless pattern formed from a square tile that can be retrieved in Inkscape by selecting the rectangle and using shift-alt-i.
Source Firkin
Remixed from a design seen in 'Burghley. The Life of William Cecil', William Charlton, 1857.
Source Firkin
A grid of squares with green colours. Since the colours are randomly distributed it is automatically seamless.
Source Firkin
Black brick wall pattern. Brick your site up!
Source Alex Parker
Prismatic Abstract Background Design
Source GDJ
Abstract Tiled Background Extended 12
Source GDJ
CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
Background Wall, Art Abstract, Block Well & CC0 texture.
Source Ractapopulous