The name tells you it has curves. Oh yes, it does!
Source Peter Chon
A pale olive green background with a seamless texture.
Source V. Hartikainen
Number five from the same submitter, makes my job easy.
Source Dima Shiper
Square design drawn in Paint.net and vectorized in Vector Magic.
Source Firkin
A pattern formed from a squared tile. The tile can be accessed in Inkscape by selecting the rectangle and using shift-alt-i.
Source Firkin
A light brushed aluminum pattern for your pleasure.
Source Tim Ward
A new one called white wall, not by me this time.
Source Yuji Honzawa
Same as gray sand but lighter. A sandy pattern with small light dots, and some angled strokes.
Source Atle Mo
A very slick dark rubber grip pattern, sort of like the grip on a camera.
Source Sinisha
This one is quite simple in design, it consists of vertical stripes layered on top of a seamless texture.
Source V. Hartikainen
Seamless pattern formed from a tile that can be extracted by selecting the rectangle in Inkscape and using shift-alt-i.
Source Firkin
A seamless light gray paper texture with horizontal double lines.
Source V. Hartikainen
First pattern tailor-made for Retina, with many more to come. All the old ones are upscaled, in case you want to re-download.
Source Atle Mo
Colourful background achieved with gradient fills.
Source Firkin
Prismatic Polka Dots 3 No Background
Source GDJ
CC0 and a seamless pattern from a tile drawn in Paint.net .
Source SliverKnight
Light gray pattern with an almost wall tile-like appearance.
Source Markus Tinner
An abstract texture of water. It's not perfect, but will do. You may download if you like it.
Source V. Hartikainen
A re-make of the Gradient Squares pattern.
Source Dimitar Karaytchev
Just to prove my point, here is a slightly modified dark version.
Source Atle Mo
Looks like a technical drawing board: small squares forming a nice grid.
Source We Are Pixel8
A seamlessly repeating background pattern of wood. The image is procedurally generated, and, I think, it's turned out quite well.
Source V. Hartikainen
Prismatic Groovy Concentric Background 3 No Black
Source GDJ
From a drawing in 'Line and form', Walter Crane, 1914.
Source Firkin