Recreated from a pattern found in 'Az Osztrák-Magyar Monarchia irásban és képben', 1882. To get the tile this is based on select the rectangle in Inkscape and use shift+alt+i.
Source Firkin
Dark squares with some virus-looking dots in the grid.
Source Hugo Loning
Super simple but very nice indeed. Gray with vertical stripes.
Source Merrin Macleod
Bright Multicolored Floral Background by Karen Arnold from PDP.
Source GDJ
Formed by distorting the inside front cover of 'Diversæ insectarum volatilium : icones ad vivum accuratissmè depictæ per celeberrimum pictorem', Jacob Hoefnagel, 1630.
Source Firkin
A huge one at 800x600px. Made from a photo I took going home after work.
Source Atle Mo
Might not be super subtle, but quite original in its form.
Source Alex Smith
Prismatic Isometric Cube Extra Pattern No Background
Source GDJ
Because I love dark patterns, here is Brushed Alum in a dark coating.
Source Tim Ward
Background formed from the iconic plastic construction bricks that gave me endless hours of fun when I was a lad.
Source Firkin
Snap! It’s a pattern, and it’s not grayscale! Of course you can always change the color in Photoshop.
Source Atle Mo
Formed from a tile based on a drawing from 'Viaggi d'un artista nell'America Meridionale', Guido Boggiani, 1895.
Source Firkin
A pale olive green background with a seamless texture.
Source V. Hartikainen
A very dark spotted twinkle pattern for your twinkle needs.
Source Badhon Ebrahim
Detailed but still subtle and quite original. Lovely gray shades.
Source Kim Ruddock
You know I’m a sucker for these. Well-crafted paper pattern.
Source Mihaela Hinayon
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