Sometimes simple really is what you need, and this could fit you well.
Source Factorio.us Collective
Seamless pattern formed from a tile that can be extracted by selecting the rectangle in Inkscape and using shift-alt-i.
Source Firkin
Seamless pattern formed from a tile that can be had by selecting the rectangle in Inkscape and using shift+alt+i
Source Firkin
A seamless texture traced from an image on opengameart.org shared by Scouser.
Source Firkin
Vector version of a png that was uploaded to Pixabay by pencilparker
Source Firkin
From a drawing in 'Line and form', Walter Crane, 1914.
Source Firkin
A series of 5 patterns. That’s what the P stands for, if you didn’t guess it.
Source Dima Shiper
Non-seamless pattern drawn in Paint.net and vectorised in Vector Magic.
Source Firkin
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
One of the few full-color patterns here, but this one was just too good to pass up.
Source Alexey Usoltsev
In the spirit of WWDC 2011, here is a dark iOS inspired linen pattern.
Source Atle Mo
From a drawing in 'Resa i Afrika, genom Angola, Ovampo och Damaraland', P. Moller, 1899.
Source Firkin
Remixed from a drawing in 'Incidents on a Journey through Nubia to Darfoor', F. Ensor, 1891.
Source Firkin
An abstract texture of black metal pipes (seamless).
Source V. Hartikainen
A grid of squares with green colours. Since the colours are randomly distributed it is automatically seamless.
Source Firkin
The tile this is based on can be retrieved by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
Just to prove my point, here is a slightly modified dark version.
Source Atle Mo
Alternative colour scheme. Not a pattern for fabrics, but one produced from a jpg of a stack of fabric items that was posted on Pixabay. The tile that this is based on can be had by selecting the rectangle in Inkscape and using shift+alt+i.
Source Firkin
It has waves, so make sure you don’t get sea sickness.
Source CoolPatterns
From a drawing in 'Resa i Afrika, genom Angola, Ovampo och Damaraland', P. Moller, 1899.
Source Firkin
This is so subtle: We’re talking 1% opacity. Get your squint on!
Source Atle Mo