Sometimes simple really is what you need, and this could fit you well.
Source Factorio.us Collective
Design drawn in Paint.net, vectorised using Vector Magic and finished in Inkscape.
Source Firkin
Same classic 45-degree pattern, dark version.
Source Luke McDonald
Nice little grid. Would work great as a base on top of some other patterns.
Source Arno Gregorian
Nasty or not, it’s a nice pattern that tiles. Like they all do.
Source Badhon Ebrahim
From a drawing in 'Cowdray: the history of a great English House', Julia Roundell, 1884.
Source Firkin
Derived from a drawing in 'Historiske Afhandlinger', Adolf Jorgensen, 1898.
Source Firkin
This one is rather fun and playful. The 2X could be used at 1X too!
Source Welsley
Seamless , tileable CC-0 texture. Created by my own, feel free to use wherever you want!
Source Linolafett
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
Dark squares with some virus-looking dots in the grid.
Source Hugo Loning
With a name like this, it has to be hot. Diagonal lines in light shades.
Source Isaac
White circles connecting on a light gray background.
Source Mark Collins
First pattern tailor-made for Retina, with many more to come. All the old ones are upscaled, in case you want to re-download.
Source Atle Mo
I love cream! 50x50px and lovely in all the good ways.
Source Thomas Myrman
From a drawing of the coat of arms of the Ottoman Empire on Wikimedia.
Source Firkin
Dead simple but beautiful horizontal line pattern.
Source Fabian Schultz
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
Black paper texture, based on two different images.
Source Atle Mo
Based from Design Kindle
The tile this fill pattern is based on can be had by using shift+alt+i on the rectangle.
Source Firkin