A very simple modular origami is the fluted diamond by Molly Kahn. In addition to being simple to make, they can also be used in many places. They make beautiful decorations, make them small enough and you can turn them into earrings or pendants or even bracelets or maybe mobiles.

These are made from 2 squares of paper, from the square base. The units have pockets and flaps and are assembled using those. This can be a bit tricky at first.

Model Details:

Model: Fluted Diamond 

Creator: Molly Kahn

Difficulty Level: Simple

Paper Ratio: Square

Paper Size: 4 inches

Modules: 2

Instructions: Origami Resource Center