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 grid of squares with green colours. Since the colours are randomly distributed it is automatically seamless.
Source Firkin
The green fibers pattern will work very well in grayscale as well.
Source Matteo Di Capua
A comeback for you: the popular Escheresque, now in black.
Source Patten
Not the most creative name, but it’s a good all-purpose light background.
Source Dmitry
Lovely pattern with splattered vintage speckles.
Source David Pomfret
The image depicts a seamless pattern which was made using stripe-like things including borders.I used OCAL cliparts called "Blue Greek Key With Lines Border" uploaded by "GR8DAN" and "daisy border" uploaded by "johnny_automatic".Thanks.
Source Yamachem
A seamless pattern from a tile made from a jpg on Pixabay. To get the tile select the rectangle in Inkscape and use shift-alt-i.
Source Firkin
An attempt for cleaning up the original image in a few steps.
Source Lazur URH
From a drawing in 'Art Embroidery', M.S. Lockwood and E. Glaister, 1878.
Source Firkin
Bigger is better, right? So here you have some large carbon fiber.
Source Factorio.us Collective
A seamless pattern created from a square tile. To get the tile, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
Prismatic Polyskelion Pattern No Background
Source GDJ
Seamless pattern formed from a tile that can be extracted by selecting the rectangle in Inkscape and using 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
Zero CC tileable bark texture, photographed and made by me. CC0
Source Sojan Janso
Made by distorting a simple pattern using the 'sin waves' plugin for Paint.net and vectorising in Vector Magic
Source Firkin
This one looks like a cork panel. Feel free to use it as a tiled background on your blog or website.
Source V. Hartikainen
Number 1 in a series of 5 beautiful patterns. Can be found in colors on the submitter’s website.
Source Janos Koos
Vector version of a png that was uploaded to Pixabay by pencilparker
Source Firkin
A seamless web texture with illustration of pale color stains on canvas.
Source V. Hartikainen
Prismatic Geometric Tessellation Pattern No Background
Source GDJ
Dark pattern with some nice diagonal stitched lines crossing over.
Source Ashton
A pale yellow background pattern with vertical stripes. The stripes are partially faded. I think this background image turned out pretty well, especially those faded stripes.
Source V. Hartikainen
The image is a remix of "edo pattern-samekomon".I changed the color of dots from black to white and added BG in light-yellow.
Source Yamachem