Because I love dark patterns, here is Brushed Alum in a dark coating.
Source Tim Ward
Seamless pattern formed from a tile that can be extracted by selecting the rectangle in Inkscape and using shift-alt-i.
Source Firkin
From a drawing in 'From Snowdon to the Sea. Striking stories of North and South Wales', Marie Trevelyan, 1895.
Source Firkin
From a drawing in 'Prose and Verse ', William Linton, 1836.
Source Firkin
To get the tile this is based on select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
From a drawing in 'Bond Slaves. The story of a struggle.', Isabella Varley, 1893.
Source Firkin
A background pattern inspired by designs seen in 'Burghley. The Life of William Cecil', William Charlton, 1857.
Source Firkin
Sharp but soft triangles in light shades of gray.
Source Pixeden
Remixed from a drawing in 'Some account of the Worshipful Company of Ironmongers', John Nicholl, 1866.
Source Firkin
Lovely pattern with splattered vintage speckles.
Source David Pomfret
From a drawing in 'Uit de geschiedenis der Heilige Stede te Amsterdam', Yohannes Sterck, 1898.
Source Firkin
A comeback for you: the popular Escheresque, now in black.
Source Patten
A grid of squares with green colours. Since the colours are randomly distributed it is automatically seamless.
Source Firkin
Here's a new gray "fabric" pattern. Use it as backgrounds for websites or for other purposes.
Source V. Hartikainen
It has waves, so make sure you don’t get sea sickness.
Source CoolPatterns
Looks like an old rug or a computer chip.
Source Patutin Sergey
Sometimes simple really is what you need, and this could fit you well.
Source Factorio.us Collective
A colourful background drawn originally in paint.net and vectorised in Vector Magic.
Source Firkin