Checkered Light Emboss #280
 Grid  CC BY-SA 3.0

Sort of like the Photoshop transparent background, but better!

Source Alex Parker

 More Textures
Zig-zag pattern 6 #2413
 Grid  CC 0

A seamless pattern based on a square tile that can be retrieved in Inkscape by selecting the rectangle and using shift-alt-i.

Source Firkin

Prismatic Polka Dots Mark II No Background@2X #531
 Noise  CC 0

Prismatic Polka Dots Mark II No Background

Source GDJ

Tessellation 15 #2223
 Dark  CC 0

The unit cell for this seamless pattern can be had in Inkscape by selecting the rectangle and using shift+alt+i.

Source Firkin

Background pattern 314 (colour 5) #1835
 Pink  CC 0

The square tile this is based on can be had by selecting the rectangle in Inkscape and using shift+alt+i

Source Firkin

Pattern generator #161
 Noise  CC 0

A simple example on using clones. You can generate a nice base for a pattern fill quickly with it.

Source Lazur URH

Wood #49
 Wood  CC BY-SA 3.0

Dark wooden pattern, given the subtle treatment. based on texture from Cloaks.

Source Atle Mo
Based from Cloaks

Background pattern 41 #262
 Fabric  CC 0

A seamless pattern the starting point for which was a 'light rays' rendering in Paint.net.

Source Firkin