For those of you who may not have seen it yet, Circuits.io and Autodesk released an online tool to design and test circuits.
It allows you to create a virtual environment using breadboards, Arduinos, and plenty more. Then you can simulate the project right in your browser window. You can even work on the coding collaboratively like you would with Google Docs.
The service is free for open-source designs (read you can't keep them private), but you'll need to pay a monthly fee to keep those designs private. I started using it to test some circuits I am trying to build to handle power distribution and other things for my multicopters.
Give it a try here: http://123d.circuits.io/circuits