Seamless tile 4 #1812
 Red  CC 0

Not strictly seamless in that opposite edges are not identical. But they do marry up to make an interesting pattern

Source Firkin

 More Textures
Prismatic Geometric Tessellation Pattern No Background #551
 Noise  CC 0

Prismatic Geometric Tessellation Pattern No Background

Source GDJ

Zig-zag pattern #2418
 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

Background pattern blue #1949
 Blue  CC 0

To get the tile this is based on, select the rectangle in Inkscape and use shift+alt+i.

Source Firkin

Prismatic Dots Background 2@2X #504
 Noise  CC 0

Prismatic Dots Background 2

Source GDJ

Colourful background #2008
 Colorful  CC 0

Made by distorting a simple pattern using the 'sin waves' plugin for Paint.net and vectorising in Vector Magic

Source Firkin