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: Fluted Diamond
Creator: Molly Kahn
Book: The Magic of Origami
Authors: Alice Gray, Kunihiko Kasahara
Difficulty Level: Simple
Paper Ratio: Square
Paper Size: 4 inches
Instructions: Origami Resource Center