Sometimes you just need the simplest thing.
Source Fabricio
Dark, crisp and subtle. Tiny black lines on top of some noise.
Source Wilmotte Bastien
Got some felt in my mailbox today, so I scanned it for you to use.
Source Atle Mo
A pattern formed from repeated instances of corner decoration 8. To get the basic tile select the rectangle in Inkscape and use shift+alt+i
Source Firkin
Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern
Source Firkin
A free seamless background image with abstract texture of green "curtain".
Source V. Hartikainen
Simple wide squares with a small indent. Fits all.
Source Petr Šulc.
Prismatic Polka Dots Mark II 3 No Background
Source GDJ
A seamless chequerboard pattern formed from a tile that can be had in Inkscape by selecting the rectangle and using shift+alt+i. Alternative colour scheme.
Source Firkin
From a drawing in 'Artists and Arabs', Henry Blackburn, 1868.
Source Firkin
A very dark asfalt pattern based off of a photo taken with my iPhone.
Source Atle Mo
A seamless pattern formed from a tile made from page ornament 22. To get the tile, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Prismatic Curved Diamond Pattern 6 No Background
Source GDJ
Prismatic Abstract Geometric Background 3
Source GDJ
This one is super crisp at 2X. Lined paper with some dust and scratches.
Source HQvectors