This one is amazing, truly original. Go use it!
Source Viahorizon
Seamless pattern formed from a tile that can be extracted 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
One more brick pattern. A bit more depth to this one.
Source Benjamin Ward
Used correctly, this could be nice. Used in a bad way, all hell will break loose.
Source Atle Mo
A comeback for you: the popular Escheresque, now in black.
Source Patten
A seamless background texture of old cardboard.
Source V. Hartikainen
A seamless canvas texture for using as background on websites. Colored in pale tones of brown.
Source V. Hartikainen
Remixed from a drawing in 'Line and form', Walter Crane, 1914.
Source Firkin
A comeback for you: the popular Escheresque, now in black.
Source Patten
Not a flat you live inside, like in the UK – but a flat piece of cardboard.
Source Appleshadow
Lovely light gray floral motif with some subtle shades.
Source GraphicsWall
Zerro CC tillable texture of stones photographed and made by me. CC0
Source Sojan Janso
A rusty grunge background for websites. Feel free to use it in your site's theme.
Source V. Hartikainen
Washi (和紙?) is a type of paper made in Japan. Here’s the pattern for you!
Source Carolynne
From a drawing in 'Cassell's Library of English Literature', Henry Morley, 1883.
Source Firkin
Prismatic Groovy Concentric Background 4
Source GDJ
Simple gray checkered lines, in light tones.
Source Radosław Rzepecki
A repeating background with dark brown stone-like texture and abstract pattern that looks like tree trunks.
Source V. Hartikainen
Remixed from a drawing in 'Maidenhood; or, the Verge of the Stream', Laura Jewry, 1876.
Source Firkin
Remixed from a drawing in 'A Girl in Ten Thousand', Elizabeth Meade, 1896.
Source Firkin
A seamless pattern formed from a square tile. The tile can be retrieved by selecting the rectangle in Inkscape and using shift-alt-I.
Source Firkin