Green Fibers #91
 Dark  CC BY-SA 3.0

The green fibers pattern will work very well in grayscale as well.

Source Matteo Di Capua

 More Textures
Retro Squares Background 4 #418
 Dark  CC 0

Retro Squares Background 4

Source GDJ

Double Lined #51
 Stripes  CC BY-SA 3.0

Horizontal and vertical lines on a light gray background.

Source Adam Anlauf

Triangles pattern #2408
 Brown  CC 0

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

Prismatic Geometric Pattern Variation 2 With Background@2X #472
 Noise  CC 0

Prismatic Geometric Pattern Variation 2 With Background

Source GDJ

Vintage pattern #1960
 Grid  CC 0

Formed from a tile based on a drawing from 'Viaggi d'un artista nell'America Meridionale', Guido Boggiani, 1895.

Source Firkin

Background pattern 19 (outline) #204
 Light  CC 0

A seamless pattern the starting point for which was a 'rainbow twist' texture in Paint.net.

Source Firkin

flower seamless pattern-remix #2445
 Fabric  CC 0

This is a remix of "flower seamless pattern".I rotated the original image by 90 degrees.This is a seamless pattern of flowers.These horizontal wavy lines are one of Edo patterns which is called "tatewaku or tachiwaku or 立湧" that represents uprising steam or vapor.

Source Yamachem

Prismatic Abstract Geometric Background 3 #270
 Noise  CC BY-SA 3.0

Prismatic Abstract Geometric Background 3

Source GDJ

Smooth Wall #27
 Light  CC BY-SA 3.0

Some rectangles, a bit of dust and grunge, plus a hint of concrete.

Source Atle Mo

Noisy #45
 Paper  CC BY-SA 3.0

Looks a bit like concrete with subtle specks spread around the pattern.

Source Mladjan Antic

Colorful Floral Background 2 #487
 Dark  CC 0

Colorful Floral Background 2

Source GDJ