A simple example on using clones. You can generate a nice base for a pattern fill quickly with it.
Source Lazur URH
Sort of like the Photoshop transparent background, but better!
Source Alex Parker
A seamless background drawn in Paint.net and vectorised with Vector Magic. The starting point was a photograph of drinking straws from Pixabay.
Source Firkin
Simple combination of stripy squares with their negatively coloured counterparts
Source Firkin
The basic shapes never get old. Simple triangle pattern.
Source Atle Mo
If you’re sick of the fancy 3D, grunge and noisy patterns, take a look at this flat 2D brick wall.
Source Listvetra
There are quite a few grid patterns, but this one is a super tiny grid with some dust for good measure.
Source Dominik Kiss
A textured blue background pattern with vertical stripes.
Source V. Hartikainen
It was called Navy Blue, but I made it dark. You know, the way I like it.
Source Ethan Hamilton
A seamless pattern based on a square tile that can be retrieved in Inkscape by selecting the rectangle and using shift-alt-i.
Source Firkin
From a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Retro Circles Background 8 No Black
Source GDJ
Luxurious looking pattern (for a T-shirt maybe?) with a hint of green.
Source Simon Meek
A car pattern?! Can it be subtle? I say yes!
Source Radosław Rzepecki
Remixed from a drawing in 'Line and form', Walter Crane, 1914.
Source Firkin
Adapted from a drawing in 'Line and form', Walter Crane, 1914.
Source Firkin
Could remind you a bit of those squares in Super Mario Bros, yeh?
Source Jeff Wall
One more brick pattern. A bit more depth to this one.
Source Benjamin Ward
Zero CC Mossy stone tileable texture, photographed and made by me. CC0
Source Sojan Janso
A free background tile with a pattern of pink bump dots. This background tile is sweet! Moreover, it's designed for use as website backgrounds.
Source V. Hartikainen
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin