Dare I call this a «flat pattern»? Probably not.
Source Dax Kieran
The first pattern on here using opacity. Try it on a site with a colored background, or even using mixed colors.
Source Nathan Spady
Zero CC tileable bark texture, photographed and made by me. CC0
Source Sojan Janso
Remixed from a design on Pixabay. To get the tile this is based on select the rectangle in Inkscape and use shift-alt-i.
Source Firkin
Orange-red pattern for tiled backgrounds.
Source V. Hartikainen
Here's a new paper-like background for free use on personal and commercial projects (this applies to all background patterns here).
Source V. Hartikainen
Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
Got some felt in my mailbox today, so I scanned it for you to use.
Source Atle Mo
The name is totally random, but hey, it sounds good.
Source Atle Mo
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
Formed from a tile based on a drawing from 'Viaggi d'un artista nell'America Meridionale', Guido Boggiani, 1895.
Source Firkin
This is a more minute version of "fishnet 01".The image depicts a seamless pattern of a fishnet with a plenty of fish.It may be a lucky charm for fishermen.
Source Yamachem
Heavily remixed from a drawing that was uploaded to Pixabay by ractapopulous
Source Firkin
You know you love wood patterns, so here’s one more.
Source Richard Tabor
Derived from a drawing in 'The Murmur of the Shells', Samuel Cowen, 1879.
Source Firkin
Prismatic Snowflakes Pattern 2 No Background
Source GDJ
A browner version of the original weathered fence texture.
Source Firkin
The unit cell for this seamless pattern can be had in Inkscape by selecting the rectangle and using shift+alt+i.
Source Firkin
Turn your site into a dragon with this great scale pattern.
Source Alex Parker
A pattern formed from a squared tile. The tile can be accessed in Inkscape by selecting the rectangle and using shift-alt-i.
Source Firkin
Background Wall, Art Abstract, Block Well & CC0 texture.
Source Ractapopulous
Remixed from a PNG that was uploaded to Pixabay by k_jprather
Source Firkin
ZeroCC tileable stone texture, edited from pixabay. CC0
Source Sojan Janso
Prismatic Rounded Squares Grid 4 No Background
Source GDJ
A seamless background pattern with impressed gray dots.
Source V. Hartikainen