One of the tricks with using an Arduino to power all of your projects is finding a simple way to reconnect dozens of jumper wires when switching from one to another. Instead of giving in and buying a new Arduino for each project, here are four items that make swapping connections simple:
Phi-Connect by Liudr Arduino and Physics Gadgets
Connect all of an Arduino's pins to a breadboard with a single cable for easy project swapping.
Aussie Shield by Rugged Circuits
Pop an Arduino in face-down and have access to 28 quick-connect (screw-less) terminals.
Quick Shield by Rugged Circuits
Instantly add 28 quick-connect (screw-less) terminals to an Arduino Uno, Duemilanove, or Mega.
And if you find yourself needing a lot of inputs and/or outputs, there is the:
Mux Shield by Mayhew Labs
Control up to 48 inputs or outputs using an Arduino or Arduino Mega.
InMojo supports the Open Source Hardware Definition v1.0