A grid of squares with green colours. Since the colours are randomly distributed it is automatically seamless.
Source Firkin
Bright Multicolored Floral Background by Karen Arnold from PDP.
Source GDJ
Prismatic Polka Dots Mark II 3 No Background
Source GDJ
Redrawn based on a drawing in 'По Сѣверо-Западу Россіи' Konstantin Sluchevsky, 1897.
Source Firkin
A background pattern with a look of rough fabric.
Source V. Hartikainen
This one needs to be used in small areas; you can see it repeat.
Source Luca
The classic 45-degree diagonal line pattern, done right.
Source Jorick van Hees
This metal background pattern resembles a metal plate with rivets. Solid rivets on a metal plate.
Source V. Hartikainen
A series of 5 patterns. That’s what the P stands for, if you didn’t guess it.
Source Dima Shiper
Prismatic Groovy Concentric Background
Source GDJ
Prismatic Abstract Line Art Pattern Background
Source GDJ
Submitted in a cream color, but you know how I like it.
Source Devin Holmes
Detailed but still subtle and quite original. Lovely gray shades.
Source Kim Ruddock
Derived from an image that was uploaded to Pixabay by mdmelo.
Source Firkin
Light honeycomb pattern made up of the classic hexagon shape.
Source Federica Pelzel
Prismatic Groovy Concentric Background 4
Source GDJ
Here's a seamless brown cork board background texture. Feel free to download or reshare if you like.
Source V. Hartikainen
Like the name says, light and gray, with some small dots and circles.
Source Brenda Lay
Recreated from a pattern found in 'Az Osztrák-Magyar Monarchia irásban és képben', 1882. To get the tile this is based on select the rectangle in Inkscape and use shift+alt+i.
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