On a large canvas you can see it tiling, but used on smaller areas, it’s beautiful.
Source Paul Phönixweiß
Prismatic Groovy Concentric Background 2 No Black
Source GDJ
CC0 and seamless wellington boot pattern.
Source SliverKnight
Adapted heavily from a JPG that was uploaded to Pixabay by Viscious-Speed.
Source Firkin
Fix side and a seamless pattern formed from circles.
Source SliverKnight
Prismatic Curved Diamond Pattern No Background
Source GDJ
A green background pattern with warped vertical stripes and a grunge look.
Source V. Hartikainen
Not the most creative name, but it’s a good all-purpose light background.
Source Dmitry
Seamless pattern formed from a tile that can be extracted by selecting the rectangle in Inkscape and using shift-alt-i.
Source Firkin
Remixed from a drawing in 'A Child of the Age', Francis Adams, 1894.
Source Firkin
A grid of squares with green colours. Since the colours are randomly distributed it is automatically seamless.
Source Firkin
Like the name suggests, this background image consists of a pattern of dark bricks. It may be an option for you, if you are looking for something that looks like a brick wall for use as a background on web pages. It's not a masterpiece, but looks pretty nice when is tiled.
Source V. Hartikainen
The unit cell for this seamless pattern can be had in Inkscape by selecting the rectangle and using shift+alt+i
Source Firkin