Seamless pattern the tile for which can be had by using shift-alt-I on the selected rectangle in Inkscape.
Source Firkin
Colorful Floral Background No Black
Source GDJ
Inspired by a pattern seen on a public domain image of a very old tile. To get the unit cell, select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Here's a dark background pattern that contains a steel grid pattern as a texture. Use it as a website background or for other purposes. It's free!
Source V. Hartikainen
More Japanese-inspired patterns, Gold Scales this time.
Source Josh Green
The name tells you it has curves. Oh yes, it does!
Source Peter Chon
A comeback for you: the popular Escheresque, now in black.
Source Patten
To get the tile this is formed from select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Nicely crafted paper pattern, although a bit on the large side (500x593px).
Source Blaq Annabiosis
Prismatic Curved Diamond Pattern 4 No Background
Source GDJ
Clean and crisp lines all over the place. Wrap it up with this one.
Source Dax Kieran
Could be paper, could be a Polaroid frame – up to you!
Source Chaos
From a drawing in 'At home', J. Sowerby, J. Crane and T. Frederick, 1881.
Source Firkin
The tile this is formed from can be retrieved in Inkscape by selecting the rectangle and using shift+alt+i
Source Firkin
From a drawing in 'Art Embroidery', M.S. Lockwood and E. Glaister, 1878.
Source Firkin
Smooth Polaroid pattern with a light blue tint.
Source Daniel Beaton
Remixed from a drawing in 'Chambéry à la fin du XIVe siècle', Timoleon Chapperon, 1863.
Source Firkin
From a drawing in 'Resa i Afrika, genom Angola, Ovampo och Damaraland', P. Moller, 1899.
Source Firkin
Remixed from a drawing in 'The Canadian horticulturist', 1892
Source Firkin
Psychedelic Geometric Background No Black
Source GDJ
A dark pattern made out of 3×3 circles and a 1px shadow. This works well as a carbon texture or background.
Source Atle Mo
Remixed from a PNG that was uploaded to Pixabay by gingertea
Source Firkin