AutoDesk has just announced the release of an electronics simulator called 123D Circuits. It allows you to play around with a virtual breadboard and an Arduino board so that you can test out your designs in real-time. They are billing it as "The best way to learn electronics without breaking your Arduino."
Take a look at the video here:
Some of the essential features include: The ability to collaborate with others in real-time, a built in Arduino code editor and a huge library of components with drag-and-drop capability. 123D Circuits has a free account option but your designs will be public. If you want to make your designs private you will need to upgrade to the paid version. The upgraded accounts cost either $12 or $25 a month depending on your needs. $12 a month gets you five private circuits, 5% off PCB orders and unlimited public circuits. The $25 a month option gives you an unlimited number of private circuits. If you'd like to know more about 123D Circuits you can check out the official website here.