Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
The first pattern on here using opacity. Try it on a site with a colored background, or even using mixed colors.
Source Nathan Spady
A seamless pattern with wide vertical stripes colored in pale yellow.
Source V. Hartikainen
Colorful Floral Background 3 No Black
Source GDJ
From a drawing in 'Friend or Fortune? The story of a strange year', Robert Overton, 1897.
Source Firkin
Prismatic Floral Background No Black
Source GDJ
No relation to the band, but damn it’s subtle!
Source Thomas Myrman
The unit cell for this seamless pattern can be had in Inkscape by selecting the rectangle and using shift+alt+i.
Source Firkin
The basic shapes never get old. Simple triangle pattern.
Source Atle Mo
A grid of squares with green colours. Since the colours are randomly distributed it is automatically seamless.
Source Firkin
The tile this is based on can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin
Adapted heavily from a JPG that was uploaded to Pixabay by Viscious-Speed.
Source Firkin
The image depicts a seamless pattern of the design which includes a stylized lotus and a stylized crane.I referred to the original image in a book which is into public domain.
Source Yamachem
Square design drawn in Paint.net and vectorized in Vector Magic.
Source Firkin
Black brick wall pattern. Brick your site up!
Source Alex Parker
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
This is a seamless pattern of regular hexagon which has a honeycomb structure.
Source Yamachem
Horizontal and vertical lines on a light gray background.
Source Adam Anlauf
I have no idea what J Boo means by this name, but hey – it’s hot.
Source j Boo
This one takes you back to math class. Classic mathematic board underlay.
Source Josh Green
Remix from a drawing in 'Ostatnie chwile powstania styczniowego', Zygmunt Sulima, 1887.
Source Firkin