Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
Prismatic Geometric Tessellation Pattern 2 No Background
Source GDJ
Prismatic Triangular Background Design Mark II 5
Source GDJ
A background pattern inspired by designs seen in 'Burghley. The Life of William Cecil', William Charlton, 1857.
Source Firkin
Prismatic Hexagonalist Pattern No Background
Source GDJ
Simple gray checkered lines, in light tones.
Source Radosław Rzepecki
The image depicts a seamless pattern of Japanese Edo pattern called "kikkou-matsu" or "亀甲松" meaning " tortoiseshell-pinetree".The real pinetree is like this: https://jp.pinterest.com/pin/500744052301065077/
Source Yamachem
Remixed from a drawing in 'Chambéry à la fin du XIVe siècle', Timoleon Chapperon, 1863.
Source Firkin
The image a seamless pattern derived from a weed which I can't identify.The original weed image is from here:https://jp.pinterest.com/pin/500744052301423641/
Source Yamachem
Almost like little fish shells, or dragon skin.
Source Graphiste
To get the tile this is based on select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Formed by distorting the inside front cover of 'Diversæ insectarum volatilium : icones ad vivum accuratissmè depictæ per celeberrimum pictorem', Jacob Hoefnagel, 1630.
Source Firkin
From a drawing in 'At home', J. Sowerby, J. Crane and T. Frederick, 1881.
Source Firkin
Formed from a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
This is the third pattern called Dark Denim, but hey, we all love them!
Source Brandon Jacoby
Remixed from a raster on Pixabay that was uploaded by ArtsyBee.
Source Firkin
Fix side and a seamless pattern formed from circles.
Source SliverKnight